I was incorrect in describing it as a 'return' address - it is 'email address' on the 'general' panel in OE and OE will not allow setup without it. It is what shows as the 'from' in an email and where a 'reply' email goes. The SMTP server is not at fault as far as I can see since it matters not which one I use and other accounts send through them ok. I still suspect a DNS problem at the domain host but they have not answered that query.