That is not true. You can upload a PGP key and all your email written to SQLite (IMAP/POP3) will be stored with your PGP key. Not plain text. SMTP is for outbound, IMAP/POP3 is for inbound.
Right, but inbound emails over POP/IMAP will be TLS encrypted. You're saying emails are encrypted at rest, but they cannot be encrypted in-flight because it's forwardemail that holds the TLS private key.