My recollection of the problem is that, for example:
hostname: host.example.net
sitename: example.com
username: example
when you log in to nutsmail as:
email: example
password: password
and you send email without doing any configuration,
the return email address in the mail you send is going to be:
example@host.example.net
and not
example@example.com
As I understand it, DA made a change to SquirrelMail to fix this, but that the problem occurs with NutsMail.
Can you test for us?
Jeff