Hi Art--
Check your SMTP settings in Tools | Accounts | edit appropriate account.
Brainstorming possibilities--your ISP won't send if the message comes from
anybody@not-the-isp's-domain. In such a case, sometimes you can set the
SMTP up to login as ISP-user to be able to send from a non-ISP email.
Some ISPs ask for a secure connection and have a specific port number that
should be used (Gmail, while not an ISP, does this to allow you to use their
SMTP server while connected through a different ISP).
Ask your ISP about how to send email that comes from a different address
(you may need to push--CompuServe has told me they don't support this (a
while back)).
If you got this account to send in Mail, check the settings there and see
how they compare. Does dot-Mac have their own SMTP server? Check the
dot-Mac instructions for setting up POP access, perhaps they do something
similar to Gmail.
Daiya