Hi,
I use MSGTAG free version too, and can't link Avast! and MSGTAG to work together, i have 2 mail addresses managed by Avast! and one more for MSGTAG.
In fact, MSGTAG listen on port 25 for outbound mail, and when Avast! start it say that port 25 is already in use.
I changed the listen port of MSGTAG (and the sending port or the corresponding mail adress in Outlook) and it's okay, I can use MSGTAG and Avast! but not at the same time (for the same mail if you prefer).
So, I can:
Outlook -> MSGTAG -> Recipient = OK
Outlook -> Avast! -> Recipient = OK
Outlook -> MSGTAG -> Avast! -> Recipient = Fail (Don't know if it's a misconfiguration, or just impossible to do)
Outlook -> Avast! -> MSGTAG -> Recipient = Fail (same as above)
Hope this can help a bit Dev team to help us

Thank you.