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What “one webmail for multiple domains” actually means
Roundcube is the web interface, not the mail system itself. It does not create users or deliver messages. The IMAP server manages accounts and folders; the SMTP server accepts outgoing mail and receives mail from other servers. A single Roundcube codebase can connect to those services for addresses such as [email protected], [email protected] and [email protected].
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The domains remain logically separate in the mail configuration even though they share hardware and software. Each complete address is mapped to a virtual mailbox or alias, and users authenticate against a shared virtual-user database rather than UNIX accounts on the VPS.
Recommended architecture
| Layer | Role | What must be consistent |
|---|---|---|
| Postfix | SMTP reception, authenticated submission and hosted-domain routing | Virtual mailbox maps, alias maps, local-domain settings and TLS names |
| Dovecot | IMAP/POP3 access, authentication and usually LMTP delivery | Virtual-user credentials, mailbox paths, ownership, quotas and password hashing |
| Roundcube | One browser-based webmail application | IMAP host, SMTP host, database, PHP runtime and host-specific settings |
| DNS | Mail routing and sender authentication for every domain | MX, SPF, DKIM, DMARC and the canonical hostname’s A/AAAA record |
| VPS and network | Public service endpoint and transport security | Static IP, PTR/reverse DNS, firewall rules, certificate renewal and provider port policy |
Postfix virtual mailbox domains let hosted addresses exist without creating a UNIX account for every mailbox. Dovecot commonly receives delivery through LMTP in this arrangement. Roundcube then connects to Dovecot over IMAP and to Postfix over authenticated SMTP.
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Plan names, identities and network access first
Choose one canonical mail hostname
Use a name such as mail.example.net as the shared endpoint for SMTP, IMAP and webmail. Create an A record for IPv4 and an AAAA record only when IPv6 is correctly routed and firewalled. Obtain a TLS certificate whose names include the hostname users and mail clients actually use.
Using the same canonical name in Postfix, Dovecot and Roundcube avoids certificate-name mismatches and makes client instructions predictable. You can still publish branded webmail names later, but they must resolve to the VPS and be covered by the certificate.
Verify VPS prerequisites
- A supported Linux distribution with security updates and a static public IP.
- Provider-controlled PTR (reverse-DNS) capability, or a support process for setting the IP’s PTR to the canonical hostname.
- Backups stored off the VPS, with a documented restore procedure.
- Confirmation that the provider permits outbound TCP port 25. Some providers restrict it, and no Postfix setting can bypass that policy.
- A firewall that exposes only the services you intend to operate.
| Port | Purpose | Typical exposure |
|---|---|---|
| 25/tcp | Server-to-server SMTP delivery | Internet-facing; required for receiving mail and normally for direct outbound delivery |
| 587/tcp | Authenticated message submission | Internet-facing; require TLS and authentication |
| 465/tcp | Implicit-TLS submission, if you choose to offer it | Optional; require TLS and authentication |
| 993/tcp | IMAPS | Internet-facing; preferred for encrypted IMAP access |
| 443/tcp | HTTPS webmail | Internet-facing; redirect or disable plain HTTP |
Do not expose administrative databases, Dovecot’s unencrypted IMAP/POP3 ports or internal delivery interfaces to the public internet unless your design specifically requires them.
Configure Postfix for virtual domains and mailboxes
Declare hosted domains separately from local system domains
For mailbox-hosting domains, use Postfix’s virtual-domain features rather than treating every address as a local UNIX user. The central concepts are:
virtual_mailbox_domains: the domains for which this server stores mail.virtual_mailbox_maps: a lookup mapping each complete recipient address, such as[email protected], to a mailbox path or a Dovecot delivery destination.virtual_alias_maps: aliases, forwarding addresses and role accounts such as[email protected].- A Dovecot delivery transport, commonly LMTP, so Dovecot applies mailbox creation, quotas and folder rules consistently.
The address map must use the full local part and domain. Mapping only alice is ambiguous when several domains contain an Alice. Keep the domain in every key and normalize the case-handling policy you intend to use.
Keep machine-local names out of virtual-domain mistakes
List the VPS hostname and other machine-local names in the appropriate Postfix local-domain settings. Postfix documentation cautions that all hostnames assigned to the machine must be accounted for; otherwise mail can loop back through the virtual transport or be rejected unexpectedly. Do not add a hosted domain to both incompatible local and virtual delivery classes.
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Use aliases deliberately
Aliases are useful for addresses such as [email protected] that should deliver to an existing mailbox. They are not a substitute for a mailbox map when a user needs independent storage and login credentials. Test forwarding destinations and consider loop prevention before enabling arbitrary aliases.
Treat catch-all addresses as an exception
A catch-all accepts every local part at a domain, including mistyped and abusive addresses. Postfix’s virtual-domain guidance warns that the mailbox store must be able to accept every address when a catch-all is configured. Unless you have a specific operational reason, explicit mailbox and alias entries are safer and produce less unwanted mail.
Set up Dovecot users, storage and authentication
Choose a virtual-user database
Use a consistent virtual-user backend, commonly SQL or another Dovecot-supported database. Store each user’s complete login identity—normally the full email address—and a password hash. The same backend must be used for IMAP authentication and for SMTP submission authentication through SASL, or users will be able to read mail but fail when sending.
Define mailbox paths that cannot collide
Include the domain in the storage path or namespace. A pattern such as vmail/example.net/alice/ and vmail/example.org/alice/ keeps equal local parts in different domains separate. Apply one dedicated mailbox UID/GID, ensure Dovecot and the delivery transport can write to the directories, and prevent the web-server account from gaining broader access than necessary.
Align delivery, quotas and password policy
- Configure Postfix’s delivery transport and Dovecot’s mailbox location to describe the same path.
- Apply quotas in one authoritative layer and make the limit visible to users where practical.
- Use a modern password-hash scheme supported by your Dovecot version; never store cleartext passwords in the user database.
- Require a full address for authentication unless a domain-specific Roundcube host intentionally supplies the domain.
- Enable only encrypted IMAP and authenticated SMTP submission for normal users.
Install Roundcube once
Meet the application requirements
Roundcube requires an HTTP server, PHP, a database, an IMAP server and an SMTP server. Install the application under the web server, create its database and point the site’s document root at Roundcube’s public_html directory. Follow the version-specific PHP and database requirements for the release you deploy.
The temp and logs directories must be writable by the web-server process. Keep them outside public download paths where your web-server layout allows, deny directory listing, and remove installer files or disable the installer after initialization.
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Set the shared IMAP and SMTP endpoints
Configure the default Roundcube settings to use the canonical mail hostname, encrypted IMAP and authenticated SMTP. A typical design is IMAPS on port 993 and submission on port 587 with STARTTLS; port 465 can be offered when you deliberately support implicit TLS. The hostname in these settings must match the certificate presented by the service.
Roundcube does not manage users. Account creation, password changes, mailbox limits and deactivation belong in the Dovecot user backend and its administration process.
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One default Roundcube configuration is enough when every user visits the same URL, every backend uses the same IMAP hostname and users always enter full email addresses. Host-specific files are useful for branded URLs, different login defaults or separate backends.
Map each webmail hostname to a small configuration file
Roundcube supports an include_host_config map. The following illustrates the documented pattern; adapt file locations and hostnames to your installation:
$config['include_host_config'] = array(
'webmail.example.net' => 'example_net_config.inc.php',
'webmail.example.org' => 'example_org_config.inc.php',
);Create the referenced files in the configuration directory and set only the values that differ. For example:
// example_net_config.inc.php
$config['imap_host'] = 'ssl://mail.example.net:993';
$config['username_domain'] = 'example.net';
// example_org_config.inc.php
$config['imap_host'] = 'ssl://mail.example.net:993';
$config['username_domain'] = 'example.org';username_domain appends the selected domain when a user enters only the local part. If a person types alice at webmail.example.net, Roundcube can submit [email protected] to the IMAP server. Decide whether this convenience is appropriate: full-address logins are clearer when the same person can access more than one domain.
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Keep SMTP submission aligned with the login identity. If Roundcube rewrites only the IMAP username but sends a different SMTP username, users may see authentication failures or an unexpected From address. Test both protocols through each hostname.
Publish DNS for every hosted domain
DNS is configured per domain even though the mail software is shared. Point each domain’s MX record at the canonical hostname, not at an unqualified server name.
| Record | Example purpose | Important qualification |
|---|---|---|
| MX | example.net delivers to mail.example.net |
Publish for every hosted domain; the target must resolve to the VPS |
| A/AAAA | mail.example.net resolves to the VPS address |
Publish AAAA only when IPv6 connectivity and filtering are correct |
| SPF | Authorizes the actual outbound sending hosts | Use the sending infrastructure you really operate; do not copy an example without replacing it |
| DKIM | Publishes the public key for the selector used by your signer | The selector, domain and key must match the signature generated in transit |
| DMARC | Policy at _dmarc for alignment and reporting |
Start with a policy appropriate to your monitoring and enforcement readiness |
| PTR | Reverse DNS for the VPS IP points to the canonical hostname | Usually controlled by the VPS provider; forward and reverse names should be coherent |
Repeat the SPF, DKIM and DMARC work for each organizational domain. A valid record on one domain does not authenticate messages sent as another domain.
Secure transport and the web application
Use certificates that match every public name
The certificate must cover mail.example.net and any public webmail aliases such as webmail.example.net. Renew it automatically where possible, monitor expiry, and reload Postfix, Dovecot and the web server after renewal according to their service procedures.
Require encryption before authentication
Disable plaintext authentication on internet-facing IMAP and SMTP submission unless it is protected inside TLS. Enforce HTTPS for Roundcube and redirect HTTP only if the web server is configured to avoid redirect loops. Do not expose database credentials or Dovecot passdb files through the web root.
Limit abuse
- Rate-limit login attempts and submission activity, and use fail2ban or an equivalent control where appropriate.
- Run spam and malware filtering before mail reaches user folders, with a quarantine and release process you can operate.
- Restrict relay permissions so unauthenticated clients cannot send through your server.
- Log SMTP, IMAP, authentication and administrative events, and retain enough history to investigate abuse without retaining unnecessary personal data.
Test each domain before switching production mail
- Resolve the A/AAAA and MX records from an external network and verify that the MX target is the canonical hostname.
- Open each webmail URL over HTTPS and confirm the certificate name, redirect behavior and Roundcube login page.
- Log in with a full address for one mailbox in every domain. If you use
username_domain, also test the local-part-only behavior on the intended host. - Read an existing message over IMAP, create a folder, send a message through authenticated submission and confirm that the Sent folder is updated.
- Send mail from an external account to each domain and verify delivery to the intended mailbox, including an alias.
- Send to an external recipient and inspect the message headers for the expected From domain, DKIM signature and authentication results.
- Test an unknown recipient and confirm that it is rejected rather than silently accepted, unless a documented catch-all is intentional.
- Restart or reload each service and repeat one login and one delivery test to catch configuration changes that were not loaded.
These checks are functional acceptance tests, not a performance benchmark. Capacity depends on message volume, mailbox sizes, filtering workload, storage and provider limits; a small VPS configuration example, such as 1 GB of RAM in a deployment guide, is not a universal sizing guarantee.
Troubleshoot by symptom
Roundcube loads but login fails
- Confirm the Dovecot username format. Try the full address and compare it with the account key in the virtual-user database.
- Check that the host-specific file was selected for the hostname in the browser and that
username_domainis not appending the wrong domain. - Verify that the IMAP hostname, port and TLS mode match the certificate and Dovecot listener.
- Read the Dovecot authentication log and Roundcube log while making one test login; distinguish an unknown user from a TLS or database error.
Login works but sending fails
- Ensure Roundcube’s SMTP host and submission port are configured separately from the IMAP host when necessary.
- Confirm SMTP AUTH uses the same credentials and identity format as Dovecot.
- Check that the client is using TLS before authentication and that the firewall allows the selected submission port.
- Inspect Postfix logs for relay denial, SASL failure, policy rejection or a sender address that is not permitted for the authenticated user.
Inbound mail is rejected or disappears
- Check the domain’s MX record and confirm that Postfix lists the domain as a virtual mailbox domain.
- Verify the exact recipient key in
virtual_mailbox_maps, including spelling and domain. - Confirm the LMTP or virtual-delivery transport can write to the Dovecot mailbox path and that UID/GID permissions are correct.
- Look for a local-domain and virtual-domain classification conflict that causes loops or rejects.
Mail reaches recipients’ spam folders
Check PTR, SPF, DKIM and DMARC alignment for the specific From domain, then review the sending IP’s reputation and message content. DNS records authenticate claims; they do not guarantee inbox placement. Ensure the VPS provider allows outbound port 25 and that the server is not an open relay.
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A certificate warning appears for one domain
Compare the hostname in the browser or mail client with the certificate’s names. Add the missing webmail alias to the certificate, or direct users to the canonical hostname that the existing certificate covers. Reload every service after renewal.
Operate the stack after launch
Mail hosting is an ongoing service, not a one-time installer. Schedule operating-system, Postfix, Dovecot, Roundcube, PHP and database updates; read release notes for configuration changes; and test upgrades on a copy when possible.
- Back up mailbox data, virtual-user records, aliases, DKIM private keys, Roundcube configuration and its database. Store at least one copy off the VPS.
- Perform a restore test, not just a backup job test. Confirm that a restored mailbox can be opened and that the restored configuration does not expose credentials.
- Monitor disk space, inode usage, queue size, delivery failures, authentication failures, certificate expiry and DNS changes.
- Set mailbox quotas before storage fills the filesystem. Keep free space for logs, queues and database operations.
- Document domain onboarding and offboarding: DNS records, mailbox creation, aliases, credentials, DKIM selectors, backups and deletion timelines.
- Have an abuse response process for compromised accounts, spam bursts and fraudulent messages. Disable credentials quickly and preserve relevant logs.
When a different architecture is a better fit
A hand-built Postfix/Dovecot/Roundcube stack gives fine control and can host many domains efficiently, but you own every update and operational decision. Compare it with a control panel or managed mail service using the same criteria rather than comparing only the webmail screen.
| Decision area | Questions to answer |
|---|---|
| Administration | Can you create domains, mailboxes, aliases and quotas without editing several unrelated files? |
| Identity storage | Is the virtual-user database documented, backed up and usable by both IMAP and SMTP AUTH? |
| TLS | How are certificates issued, renewed, deployed and monitored for every hostname? |
| Authentication DNS | Can you publish and rotate SPF, DKIM and DMARC records and investigate alignment failures? |
| Filtering | Who maintains spam and malware rules, quarantine handling and false-positive recovery? |
| Backups and migration | Can you export mailboxes and settings, restore them, and move domains without losing messages? |
| Scaling | What happens when storage, filtering CPU, queue volume or mailbox count grows? |
| Provider policy | Are port 25, PTR changes, abuse handling and support available for your region and plan? |
| Auditability | Can you review authentication, delivery and administrative logs while limiting access to sensitive data? |
Practical design decisions that prevent future migrations
Use full addresses as the canonical identity
Even if branded hostnames allow short logins, store and document users as full email addresses. This avoids collisions when two domains contain the same local part and makes exports, audits and migrations unambiguous.
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Keep Postfix maps, Dovecot settings, Roundcube configuration, database dumps and mailbox files in a backup plan that can be restored independently. A mailbox backup without its password database or alias map is incomplete.
Onboard one domain at a time
Create the virtual domain, one test mailbox, its aliases and DNS authentication records first. Verify inbound and outbound flow, then add the remaining users. This isolates mistakes and gives you a known-good template without making every domain unavailable during troubleshooting.
Do not confuse webmail consolidation with mail consolidation
One Roundcube URL can front several domains, but it still depends on the same IMAP and SMTP services. If one backend is down, all domains using it are affected. Plan maintenance windows, monitoring and recovery around that shared dependency.
Bottom line
Deploy one shared Postfix/Dovecot stack, model each domain as a virtual mailbox domain, and install Roundcube once against the shared IMAP and SMTP endpoints. Add host-specific Roundcube files only where branded hostnames or domain-specific login behavior justify them. Correct MX, SPF, DKIM, DMARC, TLS and reverse DNS records are required for every domain, while backups, filtering, rate limits, updates and provider port policy determine whether the service remains dependable after the initial setup.
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