Bruce Schneier writes:
I have recently come to the conclusion that e-mail is fundamentally unsecurable. The things we want out of e-mail, and an e-mail system, are not readily compatible with encryption. I advise people who want communications security to not use e-mail, but instead use an encrypted message client like OTR or Signal.
From: Testing the Usability of PGP Encryption Tools.
If you need robust security, take Schneier at his word.