You could try extending the timeout delay in your email program. I don't use MS Outlook (only OE) so I have no idea where you can change that. Whilst these timeout errors can relate to AVs (as they scan the inbound or outbound email before actually sending it), this however, isn't the case with you having disabled avast it continues.
As the error states you should contact your ISP and report these timeout errors. They no doubt will deny it and probably balme your AV, but you have the ammunition to counter that.