Hi @alavarre,
You've misinterpreted this part of the FAQ and maybe we should update it to be clearer on this point.
In this case "email encryption" refers to S/MIME encryption (between one end-user's e-mail client and another's). It doesn't refer to TLS encryption used by an e-mail server to protect connections to that server, as with STARTTLS in SMTP, or IMAPS, or SMTPS protocols. Let's Encrypt's ordinary certificates are fine for these uses and you don't need a separate certificate or a special kind of certificate to protect TLS sessions used for the delivery or retrieval of e-mails.
By contrast, S/MIME allows an e-mail author to encrypt the body of a message inside his or her e-mail client so that it can only be decrypted later inside the e-mail client of the intended recipient. This requires a different kind of certificate that Let's Encrypt doesn't issue, but this is probably not your intended application here.