...when it should probably give up sooner, clearly indicate 
truncation, and point users to the raw view.

<http://issues.guix.gnu.org/48151> is a >9 MiB mail.  The rendered 
HTML is >11 MiB.  Apart from being A Bit Much to send to clients, 
I get 504s most of the time.  It succeeded once.

Mailman's message limits are of no use here: it sits later in the 
chain.

Kind regards,

T G-R