Better antivirus (AV) protection?

[Damian]

> Question 1:  Is there something wrong in the configuration that does 
> not allow 7z scanning or probably an additional software library is 
> needed?

Your system lacks rar and unrar, and EPEL does not provide it any more.

> Question 1A: If a decoder fails, could amavis be explicitly configured 
> to try more decoders?

As far as I can tell, no.

> Question 2: Could the config be corrected to process 7z attachments 
> correctly?

You could give 7zip [1] a try. According to [2], 7zz integration does 
not need code changes. You'd have to deploy 7zz manually, though, as I 
don't see it in EPEL either.

> As a final note: I was misled by the fact that in the mail headers the 
> mail was reported as scanned and not as undecipherable.
>
>    X-Virus-Scanned: amavisd-new at noa.gr
>
> Question 3: How can we configure amavis to report such mails (which 
> failed to open for scanning) as undecipherable? 

You can't. This particular code path does not add the undecipherable flag.

[1] https://www.7-zip.org/
[2] https://gitlab.com/amavis/amavis/-/merge_requests/90