[root@sky postfix]# postconf -n
alias_database = $alias_maps
alias_maps = hash:/etc/postfix/aliases
command_directory = /usr/sbin
config_directory = /etc/postfix
daemon_directory = /usr/lib/postfix
data_directory = /var/lib/postfix
debug_peer_level = 2
html_directory = no
mail_owner = postfix
mailq_path = /usr/bin/mailq
manpage_directory = /usr/share/man
mydomain = kent.ac.uk
newaliases_path = /usr/bin/newaliases
queue_directory = /var/spool/postfix
readme_directory = no
sample_directory = /etc/postfix/sample
sendmail_path = /usr/sbin/sendmail
setgid_group = postdrop
unknown_local_recipient_reject_code = 550
[root@sky postfix]# hostname
sky


In /etc/postfix/main.cf the following overriden (rest defaults)

mydomain=kent.ac.uk



[andrew@sky ~]$ sendmail ar290@kent.ac.uk
Test this
is a test


^^ this is received from "andrew@kent.ac.uk", not from "andrew@sky.kent.ac.uk"


[andrew@sky ~]$ sendmail -f andrew@sky.kent.ac.uk ar290@kent.ac.uk
More Test
>> Log entry:
Jun 19 14:48:52 sky postfix/qmgr[4487]: 829B2DABC1: from=<andrew@sky.kent.ac.uk>, size=263, nrcpt=1 (queue active)
Jun 19 14:48:52 sky postfix/smtp[4492]: 829B2DABC1: to=<ar290@kent.ac.uk>, relay=mx6.kent.ac.uk[129.12.21.37]:25, delay=4.2, delays=4.1/0/0.01/0.07, dsn=2.0.0, status=sent (250 OK id=1QYIN3-0003HG-Jk)
Jun 19 14:48:52 sky postfix/qmgr[4487]: 829B2DABC1: removed



Both of the above