Normally, header recipient addresses and header and envelope sender addresses that are local are left as is. If the always_add_domain feature is defined, local addresses that lack a host part have an @ and the MASQUERADE_AS host appended (if it is defined). If MASQUERADE_AS is not defined, an @ and the value of $j sendmail macro ($j) are appended.

The form for the always_add_domain feature is:

FEATURE(`always_add_domain')

The always_add_domain feature is safe and recommended. It ensures that all addresses that are locally delivered will be fully qualified. See the allmasquerade feature (FEATURE(allmasquerade)) for a description of the risks surrounding masquerading addresses.