This pair of plugins looks up recipients (argument to the RCPT TO command) in an alias file. Recipients which are not found are immediately rejected. After all recipients are known, the aliases are recursively expanded.
An alias can expand to one or more addresses, a detail string (everything after '+' in the local part) is preserved in the expansion.
Duplicates are eliminated.
Unlike the sendmail aliases file, the aliases are complete email addresses, not just the local part.