This is likely related more to the configuration of the email server. Outlook is very picky, and as far as I know, checks the headers for all of the following:

To be fair, I'm almost certainly not going to install yet another client just for this community chat. (I know I'm not alone in this, projecting forward into the project's future.) Matrix + IRC with bridges to the other protocols seems the way to go. (I'm usually on IRC...albeit, not much this month because my plate's full.)

In T15010#1062, @avivey wrote:

Any user should be able to sign the doc at L1 Phorge Vision Statement - did you try it and got an error?

Glad to see Phabricator continuing as Phorge. Not going to lie, I had a panic attack this morning when I found out that Phacility was winding down.