How do muggles with children at Hogwarts (for example, Hermione's parents) communicate with their children while they're at school? The students are able to send owls but does that mean the parents can only send letters when their students send an owl? What if it's an emergency? What if their children never send an owl?
We know that Muggle mail can reach Hogwarts - Petunia once wrote a letter to Dumbledore, and he replied. We also know that the Dursley's were able to send Harry his Christmas presents; given their distaste for magic it is safe to assume that they would not have done so had it involved using an owl or any other magical means.
Snape, when he was a child, speculated that there are wizards working undercover in the Muggle Post Office, who would presumably intercept any mail meant for the wizarding world and pass it on by owl:
[...] "Severus saw the envelope, and he couldn't believe a Muggle could have contacted Hogwarts, that's all! He says there must be wizards working undercover in the postal service who take care of - "
"Apparently wizards poke their noses in everywhere!" said Petunia.
Deathly Hallows, Chapter 33, The Prince's Tale
(We don't have any canon evidence on this point as far as I know, but for the purposes of this question the details don't really matter and are probably subject to change anyway.)
Parents can send letters by normal post.
“You didn’t think it was such a freak’s school when you wrote to the head- master and begged him to take you.”
Petunia turned scarlet.
“Beg? I didn’t beg!”
“I saw his reply. It was very kind.”
“You shouldn’t have read—” whispered Petunia, “that was my private—how could you—?”
Lily gave herself away by half-glancing toward where Snape stood nearby. Petunia gasped.
“That boy found it! You and that boy have been sneaking in my room!”
“No—not sneaking—” Now Lily was on the defensive. “Severus saw the envelope, and he couldn’t believe a Muggle could have contacted Hogwarts, that’s all! He says there must be wizards working undercover in the postal service who take care of—”
Harry Potter and the Deathly Hallows, Chapter 33, The Prince's Tale
Although there is no evidence for this in the book, it seems to me that it would be unnecessary to have wizards working under cover for the postal service forwarding mail by owl to the wizarding world. Hogwarts could simply set up a P.O. box with the muggle Post Office and receive the mail normally. Then whoever was responsible for the P.O. box could either carry the mail to Hogwarts himself or forward it by owl.
People living in very remote and difficult access places (like Hogwarts) usually do not get they mail delivered by postmen. Instead, they need to come to the closest working post office to pick the mail addressed to them. There is no reason why this could not work for Hogwarts. This is consistent with the texts from those it is seen that Hogwarts receives letters from Muggles.
It isn't specified that muggles can't use owls to communicate with their child when he's at Hogwarts; in fact they don't use this sort of communication because they don't know about wizards in the first place. We can suppose that when they discovered this community they were allowed to use the same mode of communication (owls) if it did not imply magical practices. sorry for my english!