We use a mail encryption product (EncryptTitan) with some of our clients.
In order to trigger mail encryption, our users add the phrase "send-secure" to the end of their subject line.
In Microsoft Exchange/MS365, we have a mail-flow rule that detects that the message is being sent to someone "outside the organization" and contains that phrase "send-secure" in the subject line, and it adds a special X-HEADER (