References to the relationship between multipart/alternative and use of
Conneg have triggered a possibility that I'd like to explore:
We currently have many mailing lists that permit html content and many that
do not. Most of the time, the enforcement of that rule is entirely social,
ie, non-computer. There is no protocol support for telling senders what
content format is preferred or what format(s) are prohibited.
Conneg can fix that. Conneg permits the recipient system to state and
enforce content format "permissions".
I assume you're talking about the proposed CONNEG SMTP extension rather
than just the conneg format for describing recipient capabilities.
Conneg might be able to fix that problem. But first it is necessary to
fix conneg so that the information can be obtained by the sender's UA
(not the sender's MTA), and not just by the MTA that eventually relays the
message to the recipient's MX. Otherwise, it is expecting an unrelated
party to perform a potentially arbitrary and unspecified conversion.
For example, in the case of html to text, I've seen this done really well
and I've seen it done really badly - so badly that the resulting text was
almost unreadable. I don't think we want to give license to intermediate
MTAs to perform arbitrary conversions to messages, particularly in the
absence of any explicit authorization from senders.
Keith