D-Mail, short for DeLorean Mail refers to all the mails sent back in time by Okabe Rintarou throughout the course of Steins;Gate. The mails are generally sent back through use of the Microwave Ophone, but some also arrive from as far in the future as 2036, from the future Okabe.
Daru was the one who coined the term DeLorean Mail (while in the anime adaptation, it was Mayuri), naming it after the model of the car equipped with the time traveling machine in the movie trilogy Back to the Future. Okabe suggested a number of names for the mail, but it was Kurisu who shortened DeLorean Mail to D-Mail.
D-Mails are generally phone messages, but in one case a pager message sent back to the past is also considered a D-Mail. D-Mails can only be sent in their entirety if they are as large as 36 bytes or smaller; any additional content will be cut off. Additionally, the D-Mail will be split into at least three separate mails upon arrival in the past. However, by 2036, Okabe has managed to perfect D-Mails to the point that a large movie mail can be sent as a single D-Mail. D-Mails have the ability to change the world line by altering the actions taken by the receiver of the message; however this ability is largely hit-and-miss because the receiver's reaction to the D-Mail is generally random. In many cases the D-Mail does not work at all or has a different effect to the one intended. D-Mails can have drastic effects, such as changing Ruka Urushibara's gender or causing Moeka Kiryū to commit suicide.
D-Mails are sent through the Microwave Phone and can only be sent from devices connected to the Microwave Ophone, such as a phone or a pager. D-Mails can only be sent to someone with a phone (or a pager), so although they could hypothetically be sent back millions of years, they would not have any effect. The number of the target phone is also necessary to send the D-Mail, so they cannot be to individuals whose numbers the sender has not obtained.
By modifying a D-Mail to contain an individual's memories, compressing it with a black hole and sending it back in time, that individual can time leap into the past. This is the main principle of the Time Leap Machine.