Let me try to explain it a different way.
When you use STunnel this is what is happenning:
Mail client port 110 <-----> STunnel port 995 <------> GMail POP server
Mail client port 25 <------> STunnel port 587 <------> GMail SMTP server
There is really no value at all to this set up unless you want avast or some other function to scan your mail.
With avast scanning mail then:
Mail client port 110 <---> avast <---> STunnel port 995 <------> GMail POP server
Mail client port 25 <---> avast <---> STunnel port 587 <------> GMail SMTP server
When you are using STunnel then your mail client does not use ports 995/587 at all, your mail client uses the standard ports 110 and 25 - that is how avast is able to scan the mail stream between STunnel and the mail client.
If you stop avast from intercepting port 25 (which you did) then you are removing the ability of avast to scan all outgoing mail and it is exactly the same effect as turning off outbound scanning in the SMTP tab of the Internet Mail provider. In that case using STunnel outbound has no value and you might just as well connect your mail client directly to the GMail SMTP server using TLS.
