fetchmail -- denial of service on reject of local warning message

Matthias Andree reports:

fetchmail will generate warning messages in certain
circumstances (for instance, when leaving oversized messages
on the server or login to the upstream fails) and send them
to the local postmaster or the user running it.
If this warning message is then refused by the SMTP listener
that fetchmail is forwarding the message to, fetchmail
crashes and does not collect further messages until it is
restarted.