i’ve had to disable inbound email scanning (IMAP) and outbound SMTP scanning on the Mail Shield because it causes problems with connecting to Gmail server using thunderbird email program
none - it just wouldn’t connect reliably - asking for PWs, timing out
disabling incoming IMAP and outgoing SMTP fixed the problem instantly
the IMAP was disabled a few months ago - then the SMTP did the same more recently
and how about making the verification letters readable by humans - took me 5 tries to make them out