Smarthost
A relay server that a mail system is configured to send all its outbound mail through, instead of delivering directly to each recipient's MX host.
What it means
With a smarthost the local server does no MX lookups for outgoing mail: it hands every message to one configured relay, which delivers onward. Postfix calls the setting relayhost; Exchange Server lets a send connector forward all mail to a smart host instead of routing by DNS. Typical reasons are a network that blocks outbound port 25, the wish to send from IP addresses with an established reputation, or a policy that all mail passes a central filter or archive.
To receivers the smarthost is the sending server: its IP is checked against blocklists, it has to be covered by the domain's SPF record, and it should sign with DKIM for the domain. A hosted SMTP relay service is a smarthost offered as a product.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- Postfix configuration parameters, postconf(5)postfix.org
- Microsoft Learn: Send connectors in Exchange Serverlearn.microsoft.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes