You will also see new lines like the following in your mail log during the smtp conversation:
Jul 5 12:31:17 hostname postfix/smtpd: setting up TLS connection from your-isp.com [220.127.116.11]
Jul 5 12:31:17 hostname postfix/smtpd: TLS connection established from your-isp.com [18.104.22.168]: TLSv1 with cipher AES128-SHA (128/128 bits)
This tells you that the connection was encrypted with TLSv1 using the cipher AES128-SHA with 128 bit key.
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