OT: Mac OSX, Squirrelmail question

[Rob Becker]

We have a mail server for our students that allows them webmail access
using Squirrelmail.  Our student email addresses are in the form of
student at students.kcai.edu.  We have appropriate DNS records for mail to
get to the server, but due to some network constraints, we had to create
an additional DNS name for the webmail access.  The off campus address
for this access is squirrelmail.kcai.edu.  When students log in from off
campus and send mail through Squirrelmail, their messages appear to be
coming from squirrelmail.kcai.edu.  For example a message sent from off
campus would appear as student at squirrelmail.kcai.edu instead of
student at students.kcai.edu.  In the Mail configuration under Server
Manager on the OSX server, the server name is specified as
students.kcai.edu.  Why is the domain portion of their addresses getting
rewritten to match the external DNS name that they used to access the
Squirrelmail page?  Shouldn't the server sent mail from it's configured
hostname?  If anyone has any advice, resources, tips, tricks or answers,
I'd love to hear them.
Thanks in advance.
Rob