Something that doesn't require securing email. Both S/MIME and PGP were solutions for 1980s problems (TFA is slightly off about PGP's start date, the PGP design dates from 1987 and MSDOS, not the 1990s, and S/MIME via PEM is from 1986). They're pretty much irrelevant today because almost all email is encrypted anyway via StartTLS and if you need full end-to-end encryption you use Signal or something similar.