'ordinary mail' is correct and usable in written English.
You can use it when referring to mail sent by traditional postal service without any extra services such as tracking or express delivery. For example: "I sent the package by ordinary mail, so I'm not sure when it will arrive".
Mr. Strempler of Mediplan said his company received more orders through ordinary mail than from its Web site.
Mail that arrives without a stamp has the same chance of getting through to your inbox as ordinary mail does in the current anti-spam environment.
(e) The cost of delivery of any document by the Registration Unit by any means other than ordinary mail shall be charged to the requester at a rate sufficient to cover the expense to the Registration Unit.
Lavonda Somerville, a letter carrier, was far from hysterical as she kept to her appointed rounds in the northeast quadrant of a city where ordinary mail delivery has become something of a remarkable event.
They also transported 200 pounds weight of ordinary mail on behalf of the Fiji postal authorities.
Questionnaires were returned by ordinary mail.
But through ordinary e-mail and electronic mailing lists, political news, gossip, rumour and vitriolic abuse spread at lightning speed.
I would like to know whether these were secret ciphered telegrams or the ordinary army mail, army communications which were not secret.
The first, a brown 10 centimes stamp, could be used to send a letter up to a distance of 30 km; the blue 20 centimes could be used on all other ordinary national mail.
For the first set of samples (samples 1 33, see Table 2) faeces were collected on a Thursday and sent by ordinary regular mail without a cooling device.
The questionnaires and a stamped, addressed return envelope were distributed by ordinary postal mail, to be completed at home by the patients.
