When sendmail logs the recipient's record, it will include the identity of the controlling user, if there is one. A controlling user is set when delivering to files or through programs (Section 12.2.2):

ctladdr=<you@your.domain> (111/22),

The controlling user is printed as an address, the <you@your.domain >, although it could also be a local login name when mail originates locally (such as you). If the controlling user is the identity of a local user, the uid (the 111) and the gid (the 22) for that user are also logged.