Normally 'mail' is used by username@FQDN. Example, firstname.lastname@example.org.
AFAIK if you want it to come from a specific user, that user must be running mail. Or, you can pass the -U switch, or do a mail -f /var/mail/<username> (same as -U).
The mail editor is much like the editor 'ed' so, as to line breaks I'm going to have to say read up on ed and get to know it. I'm the heart of eVIl type of girl. In the end, it might just be simpler to pipe it all to a php script.
Forgot about mailx. It has an -r flag to do a 'from' field. Haven't tested it however.
So, umm - mailx might not be the same as the opensource mailx. AIX system 5.x and 6.x have a mailx utility that I use at work to send messages to some users so they don't see 'from root@prodserver'. Instead they see 'cpmscriptserver@prodserver'.
Hope this helps some,