[anologwintermut]

For HTTPS. I will bet you they don't use perfect forward secrecy for TLS with SMTP traffic. Not because they don't want to , but because it likely isn't supported for a huge set of the servers they connect to. A large number SSL terminators/ accelerators that e.g Microsoft uses, don't likely support it and who knows about other stuff.

[X-Istence]

They do use PFS for TLS with SMTP traffic when the server on the other end supports it.