Safe mail (10,4)

I believe the answer is:

registered post

(Other definitions for registered post that I've seen before include "Specially secure delivery" , "important letters? - with 28a" , "sign for this?" , "Special form of delivery" , "Type of mail" .)

I've seen this clue in the Sydney Morning Herald.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.