Let’s check one thing …

Are you using the avast Outlook plugin or the avast Internet Mail provider? There is a difference in the way they work.

I too am a customer of Comcast, I have no indication of any failure to receive or send mail to/from Comcast is happening for me with avast scanning the mail but my primary mail client is not Outlook.

However, there are no significant indications in the forums of problems with using current production versions of avast along with Outllook.