Why don't you use the Mail Protection Wizard (Start Menu > avast! antivirus > MPW) and choose 'manual configuration' of your email accounts? Oh, close all your email clients (programs) before running the MPW.
After that, what are your configurations (pop3 and smtp servers, user id, smtp authentication...) into your email account? They should be changed for something like this:
pop3=smtp: 127.0.0.1
user id: user#pop_address
smtp authentication on
user id: #smtp_address
The answer to your question is no, you do not have to use OE as your email client...