leaking letterbox on upvc door what is best to seal it with?

i had one that leaked but found it had been fitted back to front by the builder so i fitted it correctly

now no leaks check yours

the letterbox is held in place with two dome capped screws ..remove the backnut ..pull letterbox foreward ..apply thin bead of white silicone around slot ..push plate back and rescrew ..wipe off excessive silicone

This is why I choose have a letterbox, use a wall mounted post box, letterboxes are always going to be draughty