> Let's Encrypt is bringing the duration of certs down not up. I believe their goal is 30 minutes validity.

This is mandated by the CA/Browser Forum, which, as the name implies, is related to browsers. It says nothing about e-mail clients. In its Extended Key Usage filed (RFC 5280 § 4.2.1.12) a certificate could not have server or client authentication, but have email protection as an allowed use.

Edit: there is an RFC for "Extensions to Automatic Certificate Management Environment for End-User S/MIME Certificates"

* https://datatracker.ietf.org/doc/html/rfc8823

Let's Encrypt has always had short-lived certificates, long before they were mandated by the CAB. They've been pushing the cutting edge here