Yep. Could be a couple of things. Your ISP might require your ISP email address as the sender address on the account. That's the simple one. Usually, however, your ISP could be disallowing your use of their service to send from anything but their own SMTP. In your account settings, for outgoing mail, you might have to use your ISP SMTP instead of your DH one.
Some of our clients have this problem with their ISP's, and many of them have to do both, change the sender address to the ISP's, but put in the DH one as their return address, as well as use their ISP's SMTP for outgoing mail.
Go to Accounts on your TOOLS menu and hit properties, then servers. Change accordingly until it works.
Or, call your ISP and complain.
...At least that is what the problem sounds like to me. I could be all wet, though.
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