Hello beczesz, welcome to the forums.
In general, you have to add Avast's root certificate into mail client's trusted certificate store.
It should be automatically done but in some cases you have to do that on your own.
So...
- What is your mail client? (Outlook, Thunderbird, etc.)
- What is your avast version/edition? (19.8 Free etc.)
- What is your OS? (Windows 10 Home 64bit etc.)