Long time ago, when the concept of mail and letters came about, the letter carrier would ride up and announce delivery by playing some little "call" on his bugle like instrument. These calls were distinctive to the carrier, route, area - well distinctive and the recipient knew of the posting; this is like your "you've got mail" ditty.
I know of a dozen "post horn" inclusions in music dating from the early 18th century. Bach, Vivaldi, Mozart, Haydn.....all and a zillion others. Perhaps none so fetching as Mahler's from his 3rd Symphony...a 20th century work.
A lot has been thought about regarding this little part of a whole. I like, in particular, that he got a message...not a letter...but a message that started in some Alpine valley in times endlessly before this and it just now bubbled into hearing range...at the top of a mountain and hearing a tune from the valley....