New comment by Duncaen on mblaze repository https://github.com/leahneukirchen/mblaze/issues/218#issuecomment-974629671 Comment: This happens if there is a mail with "In-Reply-To" header to a message id that is not part of the mails passed to `mthread`, so instead of silently ignoring this, it prints the message id and uses that as parent. Sequences that are not threaded don't look at the "In-Reply-To" header so they don't add those missing message ids.