Rules that hide hosts in a domain should be applied only to sender addresses. Avoid the temptation to place such substitutions of host for domain names into rule set 3. 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.
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.
All versions of sendmail except
V8 wrongly call rule set 2 and the recipient
when processing the envelope sender.