The TLS connections are recorded in the mail logs, along with other significant actions that are related to messages, such as filter actions, anti-virus and anti-spam verdicts, and delivery attempts. If there is a successful TLS connection, there will be a TLS success entry in the mail logs. Likewise, a failed TLS connection produces a TLS failed entry. If a message does not have an associated TLS entry in the log file, that message was not delivered over a TLS connection.
Reference:
The exhibit in this Qshows a successful TLS connection from the remote host (reception) in the mail log.