Hi drahcir,
Disable https using the account settings in your mail client. Then check this list to see if avast mailshield automatically decided to provide https encryption: user interface > real time shields > mail shield > expert settings > ssl accounts. If not, add the mail host manually.
Mail shield will now do the https encoding/decoding your mail client normally performs.
If you are using webmail from within your browser, you need not turn of https encryption.
Regards