What email program are you using (assume MS Outlook {not OE}?
If MS Outlook, have you got the Outlook/Exchange provider enabled?
If MS Outlook, does the plug-in for avast show in MS Outlook?
Have you had any error messages displayed 'I can't open my messages?' If so, what are they?
Have you checked the avast help file, Resident Protection - "Outlook/Exchange" - Provider Settings