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You can send email from an IIS-hosted application either by submitting it to an SMTP server or by writing it to a pickup directory for another process to deliver. These are application delivery options; they are distinct from the legacy IIS SMTP virtual server, which Microsoft describes as a relay agent rather than a complete mail-delivery system.
Choose how the application will hand off messages
Microsoft documents two approaches for IIS application email: network delivery through an SMTP server, or pickup-directory delivery for later processing. The right choice depends on whether a mail server is available to accept messages and whether a separate process can collect files from disk. See Microsoft’s application email configuration guidance.
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| Approach | How it works | What it needs |
|---|---|---|
| Network delivery | The application submits each message to an SMTP endpoint. | An operational SMTP host and any port and authentication details that host requires. |
| Pickup directory | The application writes messages to disk for later delivery. | A suitable downstream application or user to process the stored messages. |
Network delivery is the direct route when an SMTP service is ready to accept the application’s messages. Pickup-directory delivery avoids a live SMTP submission at the moment the application creates a message, but it does not deliver mail by itself: a downstream processor must handle the files.
Configure application-level SMTP delivery
Set the sender address and choose network delivery in the application configuration. For network delivery, provide the SMTP host, port, and credentials if the host requires authentication. Microsoft’s SMTP network settings reference lists the host, port, username, password, and default-credentials settings. It gives port 25 as the default; that is a configuration default, not a promise that a particular provider accepts connections on port 25.
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- Identify the sender address the application should use.
- Select network delivery rather than pickup-directory storage.
- Enter the hostname and port supplied by the SMTP service you intend to use.
- Configure the authentication options that service requires. Do not assume anonymous submission or a particular credential method is accepted.
- Test delivery against that service and investigate any rejection using its response and configuration requirements.
There is no universal SMTP hostname, authentication method, or port for IIS. The mail provider’s requirements take precedence over the framework’s default values.
Use pickup-directory delivery when another process will deliver mail
With pickup-directory delivery, the application stores messages on disk instead of submitting them to an SMTP endpoint immediately. Configure the pickup directory in the application’s mail settings, then ensure a separate application or user is responsible for processing the messages. Microsoft’s email configuration reference describes this option. Without a functioning downstream pickup processor, files in the directory remain stored rather than reaching recipients.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Understand the legacy IIS SMTP virtual server
Some administrators use “IIS email” to mean the IIS SMTP virtual server rather than an application’s SMTP settings. Microsoft describes the legacy service this way: “The IIS SMTP service is a relay agent only.” It forwards messages to another SMTP server for delivery; it is not interchangeable with the application-level configuration described above. The legacy ASP.NET documentation identifies the component as part of IIS 6.0 and cautions against using it to relay to on-premises Exchange or Office 365. It directs those scenarios to supported Exchange Server versions. Confirm current support for the exact Windows Server version and mail service before designing a relay around this component.
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Restrict relay access
An improperly open relay can be abused to send unsolicited email. Microsoft warns that anonymous access combined with mail relaying increases this risk; its guidance recommends limiting relay to specified servers. Configure an allowlist containing only the application hosts that need to submit messages, rather than permitting unrestricted relay. Microsoft’s SharePoint outgoing-email guidance explains the relay restriction procedure in its product context.
Quick Recap
- Allow relay only from known application server IP addresses.
- Do not treat anonymous access as a reason to allow any host to relay.
- Review the configured allowlist when application servers or network addresses change.
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