I'm new to OS X and am feeling my way. My 24" iMac (my first-ever Mac) arrived on Friday. I have 3 e-mail accounts: one for business, one for personal, and one that my ISP provides. The business and personal accounts are hosted so that I can change ISP without changing e-mail addresses or having to move my accounts about. The business and personal accounts use the host's POP system for incoming mail and the ISP-provided account uses the ISP's POP system (naturally!). All 3 accounts use the ISP's SMTP system for outgoing mail. When I send an e-mail from the business or personal accounts, Mail reports that the ISP's SMTP system has rejected that e-mail address. Thunderbird has no such problems; in fact, I copied my PC settings - lock, stock, and barrell - to my iMac and set about e-mailing as if nothing had happened. I'm back to using Thunderbird for now but readily concede that I may have missed something that would enable Mail to work for me. Any helpful comments would be appreciated.