I don't think Avast knows a 10091 error.
The 10091 is normally a mailErrorNetworkNotReady (Network subsystem is not ready for communication) error.
In simple words, the mail server can not be reach. Did you allow all parts of Avast in your firewall?
What other security (related) software do you have installed?
Does the mail work if you disable the Avast Internet Mail provider?
If you need ssl or not should have been told by your mail provider.
For more information about ssl have a look
HERE