Description
The issue:
Mail transport agents can bounce an email if it violates RFCs. One limitation is line length. This is why MIME includes content encodings like quoted-printable so a long original line can be split for email and reassembled by the mail user agent for display.
fail2ban takes the output of a command and puts it in an email. The email is not MIME encoded. No checks on the length of a line are made. Depending on the MTA the email encounters on its journey, it may bounce. The Exim MTA, the default on Debian, is an MTA which bounces emails with long lines. This means the whole message fail2ban was trying to deliver doesn't reach the recipient, not the rest of it once the long lines are dropped.
message has lines too long for transport is an example bounce diagnostic.
An example triggering case is
The IP 208.65.84.108 has just been banned by Fail2Ban after
5 attempts against sshd.
because whois(1) for that IP address gives long lines.
$ whois 208.65.84.108 | awk '{print length}' | sort -nr | sed q
1192
But I don't think the issue is limited to whois's output.