Sendmail is really good and fast, but any documentation I have seen on it is horrible. I have an external relay (MimeSweeper) that I do content and Virus checking with so I shove all sendmail's outbound mail through that, which makes the config a bit simpler. This could also be your ISP's mail relay.
As MimeSweeper does all the anti-spam stuff too, I didn't need to worry about that in sendmail.cf
For your particular problem, you need to change the DM value to be whatever you want to be after the @ in your mail address
we run mail with the following options in order to check every 30 seconds:
startsrc -s sendmail -a "-bd -q30s" #change 30s to 5m for 5 minutes, etc.
Assuming domain is "mydomain.com" and hostname is "myhost"....
We only set the following in /etc/sendmail.cf and it works
.
.
.
#DwYourHostName
#DmYourDomainName
Dwmyhost
Dmmydomain.com
.
.
.
# my official domain name
Djmydomain.com
.
.
.
# "Smart" relay host (may be null)
# Relay host to forward outgoing mail not in the local domain to.
DSsmtp:relay.mydomain.com
.
.
# who I masquerade as (null for no masquerading) (see also $=M)
DMmydomain.com