so... apparently Lync is encrypted, but if your partner doesn't notice your message it's sent as a plain text "Missed conversation with..." email...

check the "Received:" headers. chances are high they run STARTTLS over the email.

@sebastian I don't think that guarantees that the email is encrypted all the way from the sender to recipient

@oskaremil It does. Think the message are encrypted all the way to Lync server. Then the Lync-SMTP establishes a STARTTLS session with your provider's server. After that, you use IMAPS or POP3S to fetch the mail. Voila, secured all the way to you.