By default, the local hostname is revealed in the Message-ID header. This poses two problems: 1. Private information leakage: if you send mail from `alias@example.net`, you don't want it known that you're sending it from YourRealNamesPC. And if you're using multiple alias addresses, you don't want your computer's hostname linking them. Mutt [stopped](https://gitlab.com/muttmua/mutt/-/commit/82973a6ea99075b822d338c63639787bf2445a41) including the User-Agent header by default for similar reasons. 2. Increase in the spam score: Rspamd (and perhaps other spam filters) increases the spam score if the Message-ID does not end with a FQDN - see the `MID_RHS_NOT_FQDN` rule. I suggest using the hostname of the `From:` address. Thunderbird does this as well. The RFC allows it: https://www.rfc-editor.org/rfc/rfc2822#section-3.6.4 > Though other algorithms will work, it is RECOMMENDED that the right hand side contain some domain identifier (either of the host itself or otherwise) such that the generator of the message identifier can guarantee the uniqueness of the left hand side within the scope of that domain.