Spurious “Invalid mailbox syntax is used” Error Message

I have used the procedure described in the docs to provide the user with a choice of mail recipients in a contact form. As instructed in the help pages I’ve inserted [your-recipient] into the To field on the Mail tab. That works fine and the email goes to the correct place.

However, I get an error message on the Mail tab saying “Invalid mailbox syntax is used.” I found that I can ignore that error and everything works OK. However, it’s confusing and the error message should probably not be displayed, right?