'Usually' but definitely not always. Account ID is also usually at the application level, while Message-ID originates lower, at the mailserver level. When the a

'Usually' but definitely not always. Account ID is also usually at the application level, while Message-ID originates lower, at the mailserver level. When the app posts something off to the mailserver, how is it supposed to get the Message-ID back? At least with the cruft, bounces are just handed back to the app to deal with.