The most common and most likely answer to this crossword clue is the 4 plus 4 letter word POST HORN
I've seen this crossword clue in The Times
P
O
S
T
H
O
R
N
More clues leading to the result POST HORN
Buddy explains!
The straight part
We know that "Caller from the Mail" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case caller from the mail = post horn.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.