The most common and most likely answer to this crossword clue is the 5 letter word SHIPS
S
H
I
P
S
Buddy explains!

The straight part
We know that "Mails" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mails = ships.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SHIPS mean?
SHIPS noun- A water-borne vessel generally larger than a boat.
- (chiefly in combination) A vessel which travels through any medium other than across land, such as an airship or spaceship.
- A sailing vessel with three or more square-rigged masts.
- A dish or utensil (originally fashioned like the hull of a ship) used to hold incense.
- The third card of the Lenormand deck.
- To send by water-borne transport.
- To send (a parcel or container) to a recipient (by any means of transport).
- To release a product to vendors; to launch.
- To engage to serve on board a vessel.
- To embark on a ship.
- To put in its place.
- To take in (water) over the sides of a vessel.
- To pass (from one person to another).
- To go all in.
- To trade or send a player to another team.
- To bungle a kick and give the opposing team possession.
- A fictional romantic relationship between two characters, either real or themselves fictional.
- To support or approve of a fictional romantic relationship between two characters, either real or themselves fictional, typically in fan fiction.