Just a thought - are you using some kind of autocomplete to fill in the email address? You might have a spelling error in the name or something that keeps popping up in autocomplete which is causing the domain to reject it.
Another possibility is that you have two (or more) email addresses listed in OE and when you reply to an email, OE is trying to use your other email provider to try to send the email. That can be checked in Tools > Acconts > Mail tab > Properties button > Servers tab. Check the SMTP setting in there.
The problem can only happen when you go to send So when you click reply it has to be the correct addrees of the original sender . Are you checjking the 'From' line it is possible a previous account is showing up in that