SMTP relay service
A hosted service that accepts mail from your applications, devices or servers over SMTP and delivers it from the provider's infrastructure.
What it means
Instead of running an outbound MTA, a business points its apps, printers or on-premises server at the provider's relay host and lets the provider handle delivery and retries. Google Workspace offers one at smtp-relay.gmail.com, authenticating clients by IP address or by SMTP AUTH; without TLS, only IP address authentication is possible.
Each service has limits worth knowing before relying on it. Google's accepts at most 100 recipients per SMTP transaction, does not accept several envelope recipients when the envelope sender is empty, and monitors relayed mail for spam. The sending domain's SPF record must cover the relay, and DKIM signing for the domain should be switched on.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- Google Workspace Admin Help: Route outgoing SMTP relay messages through Googleknowledge.workspace.google.com
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