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.)