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.

[source]