P.s. in mail
You're referring to a postscript (p.s.) in an email or letter!
A postscript is a note added at the end of a letter or email, usually indicated by the abbreviation "p.s." or "P.S.". It's a way to add a quick thought or afterthought that didn't fit in the main body of the message.
For example:
Dear [Name],
... (main message)
P.S. Don't forget to RSVP by Friday!
Best, [Your Name]
In this example, the postscript is a reminder to respond to the invitation by a certain date.