Muggle Mail was the general Wizarding term for the post system used by Muggles to send letters. Letters sent by this system generally required the presence of at least one stamp, a pre-paid form of postage. When Mrs. Weasley sent a letter to the Dursleys, she was unfamiliar with the requirements regarding this and posted her letter with stamps covering every area on the envelope not used to write the addresses. A letter also need an address to whom it is going to. While it is successful for Muggles, it isn't quite as convenient as its wizarding counterpart, owl post, which is able find an addressee regardless of their current location and only requires payment if the possessor does not own their own owl, barring special arrangements such as the owlery at Hogwarts School of Witchcraft and Wizardry. Post across international lines is also regulated by Customs.
In 1995, Nymphadora Tonks sent a letter to the Dursleys informing them that they had been short-listed for the All-England Best-Kept Lawn competition. The fictitious competition was actually a ruse to get Harry out of the house so that the Advance Guard could spirit him away to 12 Grimmauld Place.
There is no post on Sundays, to the delight of Vernon Dursley, who incorrectly believed that this restriction would prevent the arrival of anymore Hogwarts acceptance letters for Harry Potter on Sunday.