Some old/unsupported FidoNet software (NetMgr? Squish?) reportedly parse addresses from MSGID control/kludge lines in NetMail messages to determine the source and/or destination address. This is clearly a bad idea for a lot of reasons, but to improve compatibility, let's not include the message number (or an '@') in the origaddr field for NetMail messages. This does slightly increase the chance of a duplicate MSGID being accidentally generated if the "mail" base is renumbered. No change for EchoMail MSGIDs (where dupe detection and threading still benefit from the additional entropy).
For Watchman.