Okay, I can repro this case, and it’s not specific to .gov domain.
@jaming is correct, this is because notification email setting is same as reply by email address.
Let’s say I set notification email to “team@discourse.org”, now I can’t set reply by email address to “team+%{reply_key}@discourse.org”, it will generate error as per this code:
So, if the notification email is set to “team@custom.gov”, then it’s not possible to set “team+%{reply_key}@custom.gov” as reply by email address.
Techincally it’s not a bug, it’s a feature which @zogstrip added via:
https://github.com/discourse/discourse/commit/f7d2fc05244d20c05e8c747c1e6ea66331d60d05
So I would recommend adding another email (perhaps reply+%{reply_key}@custom.gov) for reply by email address setting.