• Is disabling metamta.can-send-as-user a reasonable alternative to "List-Id"?
• Why aren't you using the PhabricatorMailImplementationSendGridAdapter if you're using SendGrid? (Because of T3435?)
• The quoted-printable thing is specifically a bug/behavior unique to SendGrid, right, and has gone at least 3 years without being fixed? Are you tied to SendGrid? We've seen other long-lived/odd behavior from SendGrid (T3435, T4199) and now recommend MailGun in the documentation.

In D10278#6, @epriestley wrote:

• Is disabling metamta.can-send-as-user a reasonable alternative to "List-Id"?

Well, no :) Then the email is not from the user.

• Why aren't you using the PhabricatorMailImplementationSendGridAdapter if you're using SendGrid? (Because of T3435?)

Yes, partly. The sendgrid API is basically very limited and their answer is "use smtp".

• The quoted-printable thing is specifically a bug/behavior unique to SendGrid, right, and has gone at least 3 years without being fixed?

I must admit that I don't know whether the quoted-printable thing is a sendgrid, phpmailer, or different level problem. The one line of 'quoted-printable' is also pretty simple for us to keep in the branch until we can switch off SendGrid :)

Are you tied to SendGrid?

Not particularly :) I found it's working well for us, and has a pretty generous free tier (25k mails + incoming email forwarding, which we need)

We've seen other long-lived/odd behavior from SendGrid (T3435, T4199) and now recommend MailGun in the documentation.

I can take a look at MailGun. Not sure their free tier (10k mails / mo) is enough for us, I'll check. Thx for the pointer.

I pulled the quoted-printable bit into D10303, since another install hit this issue. Hopefully this doesn't break like 3 other mailers. :/

Awesome. So what about the rest? I'm happy with keeping this in our branch (I think it's never led to a merge problem), but I also think it's super useful to at least have documented somewhere if we don't want to put this into the main branch (it took me a day to figure out what the right solution is (after talking to some of my colleagues), so if somebody else hits this they'll probably also be wasting a lot of time on it; figuring out why mail is rejected is about the least fun thing one can work on).