you cannot send via the SAIX SMTP server is you're using an IS connection (not even if you use SMTP authentication.) When you're on the IS connection, use IS's SMTP (IIRC, smtp.isdsl.net otherwise phone them and ask) server to send (outgoing) mail.
I use my own domain for email,so why does a saix or IS adsl account have an effect on my pop3 or smtp(pop3.mydomain.co.za)...that's if i understand you correct,Masticore.
but incoming problems can be related to peering between IS and SAIX. Your pop3 host may reside on the SAIX backbone which is sometimes problematic (but shouldn't be in theory) from IS.