This bug triggered me to write my own little msmtpq solution: https://gitlab.com/lxkl/msmtpq
The design is a bit different: - Always queue the message, so the MUA never hangs. - Report errors back to the user via mail messages that are put in a separate maildir. This is similar to the warnings "could not send message for ... hours" known from traditional MTAs. - Keep a record of sent and failed messages. I public-domained the whole thing, so maybe it would make sense to include it in the Debian msmtp package. But please check my Git repository from time to time for fixes.
