Perfect forward secrecy requires interaction between the two parties. So, either you need to require both parties be online simultaneously for their first inte

Perfect forward secrecy requires interaction between the two parties. So, either you need to require both parties be online simultaneously for their first interaction, or you give up on E2E encryption, or you allow the first message to not have PFS. (After you've established two-way communication, you can use Signal's dual crypto ratchet mechanism to maintain perfect forward secrecy with offline operation.) Now, maybe that first message is the null message or just a simple low-secrecy "Hello" message, but you still need that extra initial round-trip message to establish PFS. Of course, you need to delete emails once sent and once read in order for PFS to be of much value. However, without PFS, there's really no such thing as a deleted email, just emails the FSB/NSA haven't yet rubber-hosed you for the keys yet.