You will need
  • - access to the mail server;
  • - mail client on your computer;
  • services of the programmer.
Get access to the mail server. It is almost impossible if you use online services,, and other similar. But really, if your mailbox is running on your site, which is based on control of the physical server. Turn on the system time to the desired number of hours, generate and send a letter from the past to the recipient. Put the date back.
Use email programs installed on your computer. For example, Outlook or The Bat. Unlike web interfaces they provide more opportunities for users. In particular, it is possible to vary the time of sending the message. Move the system time of the computer to the desired date, create and send an email. Don't forget to restore everything back.
Hire a hacker to write a program that allows you to securely send emails from the "past". There is nothing impossible. But so far, no one seriously took up this task. It is considered to be commercially interesting and may provoke illegal actions.
Send e-mail rather than the message in some of the social networks by rearranging in advance the time on your computer. In communications, for example, in the "Classmates", hour and date of departure is fixed according to the system block the user. So, if you are chatting with a friend from the other hemisphere, there are all sorts of "tricks" over time.
Find a friend the postman to make the same operation with paper letters. Usually needed it post marked on the envelope, when it's really important to prove that you timely sent the required statutory information. But their letter puts the person to whom it is possible to find approach. It is a great need to find individuals that provide these services for a fee. Keep in mind that such actions cannot be called legitimate.