just a quickie please

[Shannon Wheeler]

I'm not currently subscribed to a sendmail list and
     I'm leaving for vacation tomorrow so
          of course a problem will show up today.

FreeBSD 6ish
sendmail 13
SquirrelMail 1.4.13

If I log into squirrelmail, I can send mail no problem.

If user is_practitioner logs in, when sending mail she receives:

ERROR:
Message not sent. Server replied:
Requested action not taken: mailbox unavailable
550 5.1.1 <is_practitioner at mcmurraycomputer.com>... I don't know who you're 
looking for.

~~~~~~~~~~~~~~
Note that the user's logon name is: is_practitioner
but her e-mail address is: isssp_practitioner at mcaonline.ca
(FreeBSD limited me to 16 characters for the username)

note also that the reject message lists the main domain that the machine 
belongs to but her e-mail address is a seperate virtual domain.

When setting up this user, I set her shell as 'mail-only' even though no 
such shell exists... most of my other users are set for 'nologin'.
In fact, hardly any users use the webmail, especially not for sending. But 
it should work and does work for other users that I've tried.

Anyone know exactly what the problem is? otherwise, just where to look...
username vs alias problem?
mail-only shell problem?
something else?

Thank you,
Shannon Wheeler
780-790-0728