What is the issue they are having? There are a lot of settings to check, so if we had an idea what was wrong we could better point you in the right direction.
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they can receive emails but not send. So what im thinking is that the smtp.domain.com is wrong as we also use smptx.domain.com
Or alternatively its an issue with an incoming port number. Generally when people are having issues its wrong incoming/out going port numbers or wrong incoming/outgoing urls