Q:

What is the "relay:" text in sendmail SMART_HOST value used for?

What's the difference between:
define(`SMART_HOST',`relay:[1.1.1.1]')dnl

and
define(`SMART_HOST',`[1.1.1.1]')dnl

I've seen both used but don't see details in sendmail documentation on what exactly relay: means.

A:

relay:[1.1.1.1] explicitly specifies relay mailer (one of smtp based mailers).
It is mailer:host syntax.
[1.1.1.1] uses default mailer. It is relay mailer by default but it may be changed using confRELAY_MAILER.
"No mailer" version is ignored when host is listed on local email domains list ($=w) [See SMailerToTriple rule set].  It makes difference e.g. when modified relay mailer is used to relay to non standard port at localhost.

