Beginning with V8.7, sendmail offered plus addressing (Section 12.4.4) in its alias file as a means to handle special aliasing needs. Usually, the plus part is stripped from the user part of the address before final delivery. That is, mail to bob+nospam would be delivered to bob.

As new delivery programs are developed, it might become desirable to pass the unstripped address to such programs. Such a delivery program would see bob+nospam as part of its command line.

If yours is such a delivery program, you can enable this latter behavior by defining this feature:

FEATURE(`preserve_local_plus_detail')

Note that this feature should not be enabled unless you are absolutely sure your delivery program will do the correct thing. If you wrongly enable this feature, mail delivery will fail.