[Mailman-Developers] Modifications to msg

[Dan Mick]

> RFC 2822 rules on the number of specific headers a message can have.
> E.g. it can have many Received: headers, but only one Reply-To: header
> (although the latter allows for multiple addresses... go figure).

Ease of parsing.  No one but humans typically looks at Received:,
but dumb MUAs need to use Reply-To:, so complicated tricks for
accumulating a list are limited there.  (at least that's what
I'd guess at for a rationale.)