This behaviour is caused by the Mail Transport Agent (MTA) handling of
& characters in the mailbox name in
From the passwd man page:
gcos-field is the user's real name, along with
information to pass along in a mail-
message heading. (It is called the
gcos-field for historical reasons.) An
``&'' (ampersand) in this field stands
for the login name (in cases where the
login name appears in a user's real
The workaround in this case is to explicitly set the From address in the mail client.
For example, using
mutt (as suggested by @mailq):
Sending a test email...
echo "Test body" | mutt -s "Test" firstname.lastname@example.org
Seems to work fine.