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Firstly, I am not sendmail guru, there entire text books devoted to the art, so it quite a powerful and complex SMTP server. However, to my mind if you can send via one SMTP server, but not the other, then the problem is not going to be Xoops, but rather your hosts sendmail config. I used to use SMTPauth mode as well, and although I had to make a few changes I never really had any great issues with it. First point of call though is you'll need to get access to the sendmail logs to try and track down the exact reason why you are been authenticated.