How to send secure emails using sp_send_dbmail When using sp_send_dbmail to send an email on SQL Server, how do you specify that the email be sent in an encrypted format?

I found a thread discussing including the string "[SECURE]" in the subject line, but these comments are 8 years old and not backed up by any source.
I don't think SQL Server has any built-in functionality to achieve what you're after. 

One possible solution is to use a local SMTP server as a relay, so that the email is not encrypted on the initial hop (between SQL Server and the local SMTP server), but it gets encrypted leveraging the SMTP server's functionalities on the second hop (to the outbound SMTP server).