We added that feature ~1 year ago in
https://github.com/discourse/discourse/commit/197e3fd72275c551f9f16e4adc6535f91490ef16
It’s only based on the OP being a staged user. There are no restrictions whatsoever regarding PMs or any time limits.
I do think it’s the right behavior though, since most email threading is done via the title, it helps people interacting with a Discourse forum via email to keep track of the conversations.