Comments can appear anywhere in the value of an email header. The initiator is an open paren, and the terminator a closed paren. They are meant only for human readers, thus they can be safely ignored by processors. These are equivalent:
MIME-Version: 1.0
MIME-Version: 1.0 (produced by MetaSend Vx.x)
MIME-Version: (produced by MetaSend Vx.x) 1.0
MIME-Version: 1.(produced by MetaSend Vx.x)0
I don't see comments used in too many headers, but they are used frequently in the Received header:
Received: from e06smtp13.uk.ibm.com (e06smtp13.uk.ibm.com [195.75.94.109]) by
smtpksrv1.mitre.org (Postfix) with ESMTP id 655301F069B for