sendmail, 4th Edition

FEATURE(loose_relay_check)

The percent-hack is a form of address that uses the % character to route mail through one host to another. For example, the following address uses the percent-hack to relay mail through hostA for delivery to hostB:

user%hostB@hostA

The intention here is to cause sendmail to connect to the hostA host and send the message by specifying user@hostB as the envelope recipient, meaning that hostA will relay the message to hostB.

V8 sendmail no longer allows the percent-hack form of relaying without first performing two checks. First, the connected-to host, the one following the @ (hostA), is looked up in the class macro defined by the RELAY_DOMAIN mc macro, and in the access database. If the connected-to host is neither in that class nor OK’d by the access database, the message is rejected with:^[129]

550 5.7.1 Relaying denied.

If the connected-to host is OK, sendmail looks up the destination host hostB, also in the class macro defined by the RELAY_DOMAIN mc macro (The RELAY_DOMAIN mc macro on page 269) and in the access database. If the destination host is neither in that class nor OK’d by the access database, the message is rejected; otherwise, it is accepted for delivery.

In brief, for the percent-hack to work, both hosts must be listed in the class macro defined by the RELAY_DOMAIN mc macro, or OK’d by the access database, or both.

One way to list them might look like this in your mc configuration file:

RELAY_DOMAIN(`hostA.domain hostB.domain')         ← V8.9 and later

If ...