Message ID contains sender's hostname instead of the hostname of sender's email address, which is preferred for better privacy. Sylpheed and Thunderbird both create Message IDs like <some-random-looking-stuff@domain-part-of-from/sender-address>.
You can override this in the Send/Header options of the account prefs.
Ah, thanks, the setting is: "Send account mail address in Message-ID." Contrary to Sylpheed/Thunderbird it includes the whole mail address (ie also localpart@...), though.