Mail call was an event in which letters and packages would be handed out to their intended recipients.
After the USS Voyager lost communication with the Alpha Quadrant in 2374, Captain Kathryn Janeway worried about crew morale now that mail call would no longer be a regular part of their day. (VOY: "Hunters")
In 2376, Neelix delivered a letter from Reginald Barclay to The Doctor with the words "mail call". (VOY: "Life Line")