Hi,
I'm not familiar with MailEnable and I don't think there's any MailEnable agent for avast... But it would be optimal if you could try to ask the MailEnable guys for this. I mean, they probably implemented all the agents (not the AV vendors themselves).
BTW of course, you can set scanning exceptions for the on-access scanner. Double click the avast tray icon -> double click the Standard Shield provider -> Advanced page -> Exceptions.
The format is typically
c:\path\path\*
Thanks
Vlk