If I recall correctly, owls deliver only at breakfast. Who enforces lining up owls at a gate till breakfast? May students receive mail outside of breakfast time?
Wild out-of-universe guess: It simulates Muggles receiving mail at school, letters are delivered at morning when students are all assembled.
We actively see owls arrive at various times throughout the day at both Hogwarts, as well as peoples homes.
Examples of mail that arrived other times.
The clock on the wall had just chimed midnight when the portrait hole burst open. Ron appeared out of nowhere as he pulled off Harry's invisibility cloak. There was a tap on the dark window. "It's Hedwig!" said Harry, hurrying to let her in. "She'll have Charlie's answer!"
Aunt Petunia was just passing around a box of after-dinner mints when a huge barn owl swooped through the dining room window, dropped a letter on Mrs. Mason's head.
Silhouetted against the golden moon, and growing larger every moment, was a large, strangely lopsided creature, and it was flapping in Harry's direction.
Examples of official mail that arrives at breakfast.
Harry heard from Hogwarts one sunny morning about a week after he had arrived at the Burrow. "Letters from school," said Mr. Weasley, passing Harry and Ron identical envelopes of yellowish parchment, addressed in green ink. "Dumbledore already knows you're here, Harry - doesn't miss a trick, that man.
The owl then fluttered onto the floor and began to attack Hagrid's coat. "Don't do that." Harry tried to wave the owl out of the way, but it snapped its beak fiercely at him and carried on savaging the coat. "Hagrid!" said Harry loudly. "There's an owl." "Pay him," Hagrid grunted into the sofa. "What?" "He wants payin' fer deliverin' the paper.
As the owls flooded into the Great Hall as usual, everyone's attention was caught at once by a long, thin package carried by six large screech owls.
So While it appears while typically official mail tends to arrive during the breakfast window, mail does not have a required time to arrive.
A side note: it does appear that usually Hogwarts halts package delivery till breakfast, as seen by this note from McGonagall that was written and placed on the package before breakfast.
Harry ripped open the letter first, which was lucky, because it said:
DO NOT OPEN THE PARCEL AT THE TABLE.
It contains your new Nimbus Two Thousand, but I don't want everybody knowing you've got a broomstick or they'll all want one. Oliver Wood will meet you tonight on the Quidditch field at seven o'clock for your first training session. -Professor McGonagall (Emphasis mine)
UPDATE- from Order of the Phoenix.
Percy sends a letter that arrives at 11 pm, and specifically states he sent this letter to arrive after the usual morning post. Whether this was simply by sending it later, OR but telling his owl to not join the usual post.
But I want to give you more than congratulations, Ron, I want to give you some advice, which is why I am sending this at night rather than by the usual morning post. Hopefully, you will be able 'o read this away from prying eyes and avoid awkward questions.
Harry often receives mail at other times, so to answer your second question, yes, students may receive mail outside of breakfast time.
I would speculate that most mail which comes through the night gets delayed until breakfast.
Since owls probably don't wake the recipient up in the middle of the night to deliver a normal letter, it makes sense that in the morning there would be an influx of owl post. Additionally, owl subscription services deliver in the morning (Hagrid receives one at the beginning of the first book). These two factors may contribute to why breakfast time is probably the most common time of day to receive mail. That said, it is received at other times too.
Yes, students may recieve mail at other times, however; if the owl comes in the night, they may not go inside the dormitories, instead, they will be kept to wait until morning. If the owl arrives during the day, it is advised to wait until the next morning.
This is pure speculation, but it's possible that there is some sort of nation wide mail magic which assists the owls at certain times of day, making mail delivery less stressful for the birds.
