sendmail, 4th Edition

Rules that hide hosts in a domain should be applied only to sender addresses. Avoid the temptation to place such substitutions of hosts for domain names into the canonify rule set 3. The canonify rule set 3 applies to all addresses and can wrongly change a nonlocal address.

Not all configuration files focus with user <@domain >. IDA, for example, uses a more complex focus: <@domain ,..., user >. Be sure you understand the style of focusing that is used in your configuration file before attempting to create new rules. ^[7]

^[7] Eric says that focusing might go away entirely in a future release because it is no longer needed under updated route-addr semantics, which discard the route part (Section 19.3.3), thereby guaranteeing that everything after the @ is the host part.

Avoid confusing rule sets 1 and 2 when adding rules. Rule set 1 is for the sender; rule set 2 is for the recipient.

Typos in rule-set declarations can be difficult to locate. For example, S1O (in which the last character is the capital letter O) will silently evaluate to rule set 1 when you really meant rule set 10.