We typically edit email templates by going to Configuration --> Email Templates. We cannot find where to edit the Temporary User Account Email Template (see attachment) How do we edit the Temporary User Account Email Template? We are testing v3.5.3
On the same place like the other email templates. Edit "temporary_user" (2nd from the top) in "Admin > Configuration > Email Templates". Press "Copy default template" and modify the parts you need to change.
Sorry but the template you mention (see attached) is the email received when a user clicks the "Resend Validation Email" button. We have updated this template already (note the highlighted part). What we meant is the Temporary User Account Email email that gets sent along with the Message email (see first attachment). Note that we have the "Pre-Deliver Temporary User Account Emails when a Secure Message is sent." setting checked on.
aha ok, then check if you can customize "message_temporary_user" template, this should be the one you required.