That does help, the fact that it worked before. So what AV were you using then ? if not theirs or McAfee's then it shouldn't have worked according to their compatibility statement.
Well avast monitors the email ports 110, 25 and 143, now if the SMTP port is used for encrypted email avast will try and scan the content of the traffic and since it is encrypted will fail I believe, now I don't know what happens after that.
If Sympatico/ISP were to use port 465 for SSL SMTP traffic in line with standards, then avast wouldn't scan it as it isn't on the monitored email ports. Whilst this doesn't directly help you you could elect not to scan outbound email, unfortunately that would apply to all outbound email traffic.
My ISP claims that it stops working when anything but approved AVs are used because they are not compatable with their services ( the only supported they say are theirs and mcafee)but the problem is even with providers off I can't send now but I can receive.
The fact that with the Internet mail provider terminated yet you couldn't send, kind of blows a hole in their statement about compatibility.
I'm sorry I'm at a loss as to what else to suggest.