I don't know if having both antivirus tools may be related or not to the original problem.
Even if you didn't use both tools at the same time, some remnants of any of them may conflict with the other one.
In any case, I would suggest reading the respective information about how to get rid of remnants after uninstalling any/both security tools.
Also, for me, it was not so clear if the problem is related to the email client, or to avast mail shield. Had you tried stopping only the mail shield of avast? What about other shields? Well, now such suggestions are of no use, since avast is no longer being used.
Again, in any case, read the info for the remnants and run each respective removal utility that applies to tools you are not using anymore (but uninstall first).