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Configure Sendmail as a smart-host client of smtp.gmail.com. Use port 587 with STARTTLS (the usual choice) or port 465 with implicit SSL/TLS. Gmail requires SMTP authentication: use the complete Gmail or Google Workspace address with a dedicated app password when the account supports app passwords, or use OAuth 2.0 through a Sendmail integration that supports XOAUTH2 or OAUTHBEARER. For organization-wide devices and applications, Google recommends its separate SMTP relay service rather than a single mailbox relay.
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The practical sequence is: prepare the Google account, select the TLS port, set Sendmail’s SMART_HOST, add a protected authinfo map entry whose key exactly matches smtp.gmail.com, configure CA trust for certificate verification, rebuild the Sendmail configuration, reload the service, and inspect the mail log while sending a controlled test.
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Have administrative access to the Sendmail host, the platform’s .mc source file, the account that will authenticate to Gmail, and the location of the host’s trusted CA bundle. You also need outbound DNS and TCP access to the selected Gmail port. The exact paths, service names, ownership rules, and configuration-build commands differ among Linux and Unix distributions, so use the Sendmail files and service tooling supplied by your operating system rather than copying a distribution-specific command blindly.
- A Gmail or Google Workspace mailbox permitted to authenticate through SMTP.
- Two-Step Verification and a dedicated app password if you are using Google’s app-password method.
- OAuth 2.0 credentials and a Sendmail SASL mechanism that supports XOAUTH2 or OAUTHBEARER if you are using OAuth instead.
- Sendmail built with TLS support, plus a current CA file or hashed CA directory.
- Permission to protect the authentication map and generated Sendmail configuration from other users.
Google documents the Gmail SMTP service, authentication requirements, and protocol choices in its Workspace SMTP guidance and Gmail IMAP and SMTP documentation.
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Choose direct Gmail SMTP or Workspace SMTP relay
Direct Gmail SMTP is appropriate when one mailbox should send authenticated mail for a host or a small, clearly bounded deployment. The credentials identify that mailbox, and Gmail applies the account’s security, sending, and anti-abuse policies. Google lists a 2,000-message-per-day limit for the Gmail SMTP server; treat that as a documented service limit, not as a promise that every account can safely send at that rate.
For printers, monitoring systems, application servers, and other organization-wide devices, Google recommends the SMTP relay service. Its fully qualified host is smtp-relay.gmail.com, and Google documents ports 25, 465, and 587 for that service. Relay rules can be managed for an organization instead of embedding one user’s mailbox password on every device.
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Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →| Decision factor | Direct Gmail SMTP | Google Workspace SMTP relay |
|---|---|---|
| Relay host | smtp.gmail.com |
smtp-relay.gmail.com |
| Typical scope | One mailbox or a small authenticated deployment | Organization devices and applications |
| Authentication model | Account credentials with an app password, or supported OAuth 2.0 | Workspace relay policy; Google documents organization-level options such as IP-based authorization |
| Documented ports | 465 for SSL; 587 for TLS | 25, 465, and 587 |
| Operational ownership | Mailbox security, app-password rotation, and account policy | Workspace administrator’s relay rules and organization controls |
| Best fit | A Sendmail host acting for a specific account | Many devices or applications that should not share a mailbox secret |
The rest of this guide covers direct smtp.gmail.com delivery. Move to the Workspace relay section when the deployment is larger, device-oriented, or likely to approach Gmail’s account-level limits.
Prepare the Google account securely
Use an app password when that method is available
Turn on 2-Step Verification for the Google account, then create an app password dedicated to this Sendmail host. Give the credential a name that identifies the server and purpose so it can be revoked without affecting interactive sign-in. Store the generated value as a secret; do not put a normal account password in a world-readable file, a public repository, or the article’s configuration examples.
The Sendmail login is the complete address, such as [email protected], not merely the local part before the at-sign. Enter the generated app password in the protected authentication map. If an administrator later revokes the app password, Sendmail will begin returning authentication failures until a replacement is created and the map is rebuilt.
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Use OAuth 2.0 where the integration supports it
Gmail protocols support OAuth 2.0. A Sendmail deployment must have a SASL path that can obtain and refresh tokens and use the mechanism expected by Gmail, commonly XOAUTH2 or OAUTHBEARER. OAuth is the standards-based alternative when app passwords are unavailable or prohibited by account policy, but it is not a drop-in substitution for a password string: token acquisition, storage, refresh, and scope approval all have to be implemented by the selected Sendmail integration.
Do not configure a normal Google account password as a workaround. A rejected password can indicate that the account requires an app password, that the app password was revoked or mistyped, or that the deployment’s OAuth mechanism does not match the account flow.
Pick the Gmail transport and port
| Port | Security model | Sendmail behavior | When to choose it |
|---|---|---|---|
587 |
STARTTLS | Connect in SMTP, issue STARTTLS, validate the certificate, then authenticate | Default choice when the Sendmail TLS build and CA trust are available |
465 |
Implicit SSL/TLS | Negotiate TLS before SMTP commands are exchanged | Use when the Sendmail integration is designed for implicit TLS and your network permits the port |
Google maps port 465 to SSL and port 587 to TLS in its current protocol guidance. Do not treat the two ports as interchangeable settings: STARTTLS requires Sendmail to issue and complete the protocol upgrade, while port 465 requires TLS from the first connection bytes.
Whichever port you select, certificate verification and authentication still matter. Encryption without peer verification can leave a client vulnerable to an impersonated endpoint, and authentication without an encrypted channel can expose credentials.
Set Sendmail’s smart host
Edit the platform’s m4 source
In the Sendmail .mc source used by your operating system, define Gmail as the smart host:
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define(`SMART_HOST', `smtp.gmail.com')dnlSMART_HOST tells Sendmail to route outbound mail through the named relay rather than attempting direct delivery to each recipient domain. The definition belongs in the source file that generates sendmail.cf; editing the generated file directly is likely to be lost at the next rebuild.
Account for the selected port
The smart-host definition identifies the relay; your installed Sendmail configuration must also express whether the connection uses STARTTLS on 587 or implicit TLS on 465. The exact m4 options and feature names vary by Sendmail release and distribution. Read the local Sendmail cf documentation for the TLS and relay syntax supported by that installation, and verify that the resulting configuration points to the same hostname used by the authentication map.
A mismatch such as configuring smtp.gmail.com as SMART_HOST while keying authentication for a different alias can produce a relay-denied or unauthenticated result even when the account credentials are valid.
Add SMTP AUTH with Sendmail authinfo
Enable the authinfo feature
Sendmail’s authinfo feature supplies SMTP AUTH data to the relay. A generic m4 pattern is:
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FEATURE(`authinfo', `hash -o /etc/mail/authinfo.db')dnlThe file name, map type, directory, and build command are distribution-specific. Use the map format documented in the Sendmail cf/README shipped with your version, and make the source and database readable only by the account and group that require them.
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Match the relay hostname exactly
The authinfo map key must match the relay hostname that Sendmail uses. For direct Gmail SMTP, that key is smtp.gmail.com. A generic entry pattern, shown with a non-functional account and secret, is:
AuthInfo:smtp.gmail.com "U:[email protected]" "P:APP_PASSWORD" "M:PLAIN"Use the full Gmail or Workspace address after U: and the generated app password after P:, or use the OAuth mechanism supported by your deployment. The example is a pattern, not a credential: never publish a real secret, commit one to source control, or leave the plaintext source file broadly readable.
Sendmail warns against using plaintext mechanisms such as PLAIN or LOGIN unless a strong encryption layer, such as STARTTLS or an external SSL tunnel, is active. Therefore, do not enable PLAIN or LOGIN on an unencrypted connection. With port 587, ensure STARTTLS has completed and the certificate has been verified before SMTP AUTH is attempted.
Build and protect the map
Generate the authentication database using the map-building procedure supplied by your operating system, then set ownership and permissions appropriate for Sendmail. Protect both the source map and the generated database; a database that is secure while its source file remains world-readable still exposes the credential. If you rotate an app password, update the source, rebuild the map, and reload Sendmail so new deliveries use the replacement.
Configure CA trust and TLS verification
STARTTLS is not complete merely because the server advertises it. Sendmail needs a TLS-capable build and a trusted CA bundle to authenticate Gmail’s certificate. Configure the appropriate CA file and/or hashed CA directory through the Sendmail TLS settings, commonly confCACERT and confCACERT_PATH.
Install the CA bundle maintained by your operating system, then point the m4 configuration at the actual file or directory on that host. Check path spelling, readability by the Sendmail process, and the directory’s hash layout when a hashed directory is used. A missing bundle, an incorrect path, or an outdated trust store can produce a certificate error before Gmail ever evaluates the account credentials.
Do not solve a certificate failure by disabling verification. First confirm that Sendmail was built with STARTTLS support, that the selected port matches the TLS mode, and that the configured CA location contains the issuer chain needed to validate the peer.
Rebuild, reload, and perform a controlled test
- Save the edited
.mcsource and the protected authinfo source. Check that the smart-host value and authinfo key both readsmtp.gmail.com. - Regenerate
sendmail.cfwith the operating system’s supported m4/build process. Do not assume a path or command from another distribution. - Rebuild the authinfo database if the source changed, then verify ownership and restrictive permissions on the source, database, and generated configuration.
- Reload or restart the Sendmail service using the service manager provided by your platform.
- Send one controlled message to an address you can inspect. Avoid beginning with a bulk job; a single test makes the first failure easier to classify.
- Immediately inspect the mail log. Look for DNS resolution, TCP connection, STARTTLS negotiation, certificate verification, SMTP AUTH response, Gmail policy response, and any quota or throttling code.
A successful transaction should show a connection to the selected Gmail endpoint, a completed TLS negotiation for the encrypted mode, accepted SMTP AUTH, and a queued or delivered message. The exact log wording depends on the Sendmail release and platform logging configuration.
Diagnose common failures
| Symptom | Likely cause | What to check and correct |
|---|---|---|
| Authentication rejected | Normal account password, invalid or revoked app password, 2-Step Verification or app-password policy, or an OAuth mechanism mismatch | Confirm the complete account address, create a fresh dedicated app password when allowed, or validate the OAuth token and SASL flow. Rebuild the authinfo map after any change. |
| STARTTLS or certificate error | Missing CA bundle, incorrect confCACERT or confCACERT_PATH, unreadable trust store, or a Sendmail build without the required TLS support |
Verify the TLS-capable build, the selected port’s TLS mode, the CA path, file permissions, and the host’s current CA bundle. Keep certificate verification enabled. |
| Relay denied or unauthenticated relay | The authinfo key does not exactly match smtp.gmail.com, the smart host differs from the map key, or the relay policy does not permit the request |
Compare the generated configuration and map entry character for character, including the hostname, then rebuild and reload. |
| Connection timeout | Outbound firewall policy, blocked SMTP port, DNS failure, or a routing problem | Check DNS resolution for smtp.gmail.com and test outbound TCP access to the selected port from the Sendmail host. If the network blocks one port, use the other only if its TLS mode is correctly supported. |
| Messages throttled or rejected after initial success | Gmail quota, spam or policy filtering, account security enforcement, or an organization rule | Review the log and account notices, keep volume within Google’s documented limits, and consider Workspace SMTP relay for device and application traffic. |
| TLS succeeds but credentials are exposed in configuration | World-readable authinfo source or database, backups containing the secret, or a secret committed to source control | Restrict ownership and permissions, remove exposed copies, rotate the app password, and treat the old credential as compromised. |
Understand limits and operational trade-offs
Direct Gmail SMTP ties Sendmail’s ability to deliver to the health and policy of one Google account. A password rotation, app-password revocation, 2-Step Verification policy change, suspicious-login challenge, or account suspension can stop all mail using that identity. Keep the mailbox ownership clear, document who can revoke or replace the credential, and monitor the Sendmail log for authentication failures rather than discovering the outage from an application queue.
The Google-listed 2,000-message-per-day Gmail SMTP limit is especially important for alerts, scheduled reports, and automated workflows. It is a service limit associated with Gmail SMTP guidance, not a target to plan around without considering recipient behavior, spam controls, and other account policies. If multiple hosts send through one mailbox, their combined activity affects that account’s operational headroom.
Workspace SMTP relay changes the administrative model. Instead of placing one mailbox credential on each device, an administrator defines which systems may relay and how they authenticate. Google identifies this service as the recommended option for devices and apps and documents smtp-relay.gmail.com with ports 25, 465, and 587. It is generally the better fit for printers, monitoring fleets, line-of-business applications, and other organization-managed senders.
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- In the Google Workspace administration environment, create or review the SMTP relay rule for the organization’s sending systems.
- Choose the rule’s authentication and source restrictions according to the network design and security policy; do not assume that a rule for one host authorizes every device.
- Change Sendmail’s smart host from
smtp.gmail.comtosmtp-relay.gmail.com. - Select one of the relay service’s documented ports and configure its corresponding TLS mode. Port 25, when allowed by the network, is not the same as authenticated submission on 587; follow the relay rule and Sendmail TLS requirements.
- Remove direct-mailbox credentials from hosts that no longer need them, rebuild the Sendmail configuration, and test one sender before migrating the rest.
- Monitor relay logs and Workspace controls for rejected sources, policy violations, and volume changes.
The relay service does not eliminate the need for transport security, source authorization, or logging. It changes where authorization is administered and can reduce the number of mailbox secrets that must be distributed.
Security and maintenance checklist
- Use a dedicated app password rather than a primary Google account password.
- Prefer OAuth 2.0 when the Sendmail integration can securely support token issuance and refresh.
- Use STARTTLS on port 587 or correctly configured implicit TLS on port 465.
- Keep CA trust configured through
confCACERTand/orconfCACERT_PATH; do not disable certificate verification to hide an error. - Ensure the authinfo key exactly matches the hostname in
SMART_HOST. - Restrict permissions on authinfo source files, generated databases, backups, and Sendmail configuration.
- Rotate or revoke credentials when a server is retired, a staff role changes, or a secret may have been exposed.
- Review logs after configuration changes and during scheduled or alert-heavy sending periods.
- Use Workspace SMTP relay when many devices or applications share an organizational sending function.
Configuration outcome
A correct direct setup has four observable properties: Sendmail routes through smtp.gmail.com; the selected port’s TLS mode is actually negotiated; Gmail receives the complete account identity and valid app-password or OAuth credentials through a protected authinfo configuration; and the host validates Gmail’s certificate against a trusted CA bundle. If any one of those properties is missing, fix that layer before changing unrelated mail settings.
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