SMTP Relay

An SMTP relay is a mail server that takes a message submitted by a client, device or application and passes it to the next server instead of delivering it itself. A relay that uses SMTP authentication is often called a submission service.

Why it matters

Applications and printers rarely run a full MTA. They hand mail to a relay that holds proper reverse DNS, authentication records and a good sender reputation. Hosted relay services also provide logs and bounce handling. The relay must restrict who may use it: an open relay, which forwards mail for anyone, is abused by spammers and leads to blocklisting. Relays are typically limited by IP address, credentials or both.

Example

Application configuration for an authenticated relay:

SMTP_HOST=smtp.relay.example.net
SMTP_PORT=587
SMTP_ENCRYPTION=starttls
SMTP_USER=app-mailer
SMTP_PASS=********

The relay signs outgoing mail with DKIM for example.com, and the domain's SPF record includes the relay's sending addresses.

Common mistakes

• Allowing relaying from entire network ranges, leaving an open relay.
• Forgetting to add the relay to the domain's SPF record.
• Hardcoding relay passwords into source code.

Related

• SMTP
• MTA
• Transactional Email
• DNSBL
• टूल: Port checker

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