Normally the LUSER_RELAY mc configuration macro (Section 4.5.6) causes the domain part of recipient addresses to be replaced with the value given to the LUSER_RELAY macro. If this behavior is undesirable, you can define this preserve_luser_host feature to correct it:

FEATURE(`preserve_luser_host')

With this feature defined, the recipient hostname is preserved. But note that it is preserved only for delivery agents that take a hostname. The default local delivery agent does not.