http://www.rfc-editor.org/rfc/rfc8314.txt
RFC 8314: Cleartext Considered Obsolete: Use of Transport Layer Security (TLS) for Email Submission and Access, K. Moore, C. Newman2018-01-01T23:00:00-00:00This specification outlines current recommendations for the use of Transport Layer Security (TLS) to provide confidentiality of email traffic between a Mail User Agent (MUA) and a Mail Submission Server or Mail Access Server. This document updates RFCs 1939, 2595, 3501, 5068, 6186, and 6409.