Reply-all is handled poorly by Phabricator, and creates a confusing experience for users. (who get the same message once from the sender, then maybe again slightly later by Phabricator)
We should send users an autoresponder reminding them to only reply-to-sender. (while still processing their mail, of course!)
Can we look at fixing the issue instead? Nagging users on etiquette seems not scalable.
Notably, with metamta.one-mail-per-recipient, "Reply All" is the same as "Reply" (other recipients are not included on the mail). This is the default setting, is more secure, better respects policies, and better respects user preferences. A large class of problems are intractable if this setting is disabled.
The only real reason to to disable this setting is if you use external mailing lists, which is presumably why it is disabled in this case. A possible approach would be to stop using external mailing lists (for example, replace them with projects), or accept that users who are on lists and are also direct recipients will receive two copies of some mail. Doing the former might be easier after T3980, since you could conceivably bind project membership to list membership.
Thanks for the pointer. We CC external mailing lists on most objects because we use Google Groups as the source of truth for group membership, and it's impossible to keep everything else synchronized with it. T3980 sounds interesting.
I'm generally very hesitant to "nag" users, and this conflicts with T7477 (which wants email to a group of users, including an application email address, to work). Of the two, I think T7477 is significantly more valuable. And, some day, many years from now, I'd theoretically like to make metamta.one-mail-per-recipient the only supported mode, which fixes this anyway.