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.
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 seperate 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 Ophone 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.