In all other antivirus packages mail virus scan installs easily - with Avast for Mac you have to fiddle around with ports and certificates as far as I understand.
That means it will not be used as a result - especially Mac users are used to more user friendly software.
You should really consider to make it easy to setup. I’m an experienced PC and Mac user and I don’t bother to take the effort to install it the way it works now - there is not even a manual for installtion.
You understand it wrong. The setup is the same like in the Windows version of avast!
Have a look on the corresponding FAQs:
https://support.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=842
https://support.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=917 (video)
The only difference is, that we do not have such FAQs for the Mac version (Apple Mail and avast! for Mac preferences) yet.
Well, about time to create it, imo. 
So I’ve been messing with the settings in mac mail, and this is what seems to work:
For a Gmail account, in mail preferences, on the accounts tab click the account and then click on the advanced tab. Uncheck the “Use SSL” box, and then for Authentication choose “External (TLS Client Certificate)”.
Then, in Avast preferences, under the “Shields” tab, click the Mail Shield “advanced” button. General should only have one box checked, the “Enable IPv6”, then under SSL, add imap.gmail.com and pop.gmail.com.
The only other thing that might make a difference is to check the box in Mac Mail, under the accounts tab, under “Account Information” for “Outgoing Mail Server (SMTP)” choose Gmail, and check the “Use only this server” box.
I hope that works for you.