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
