There was a good and detailed explanation to this question https://www.reddit.com/r/anime/comments/43b21c/spoilers_steinsgate_answers_to_frequently_asked/ :
The reason he can’t just send a D-mail to himself telling him to just not send any D-mails in the first place is because once he sent each of the D-mails, the worldline shifts to a point where someone received the D-mail but no one actually sent it in the first place. If you notice, even in episode 1 Okabe looks at Daru’s phone are see’s the received D-mail but when he checks his phone, there is no outgoing message. This is because once the D-mail is sent, it changes to a worldline where the desired outcome already happened so there was no need to send a D-mail in the first place in the new worldline. Here’s a shitty little illustration to better get my point across:
Suppose the arrow is a timeline of any scale going from past to future.
Worldline X:
----Undesired outcome----Send D-mail to before undesired outcome---->
This causes a wordline shift to a new worldline, let’s call it Worldline Y. In worldline Y, this is what the timeline looks like:
----Recieved D-mail----Desired outcome occurs---->
So here we see that from worldline X, someone can send a D-mail and diverge into worldline Y, however, once you are in worldline Y you have no record of a D-mail being sent, just of one being received. So how do you reverse a D-mail? You can’t. You can only nullify its effect. During the show, every “reversal of a D-mail” was actually a reversal of the effect caused by the D-mail by either making the recipient believe the D-mail they got was either some kind of spam (Ruka’s case) or a demand/request that was the opposite of what the initial D-mail was (Faris, Moeka). The only way the D-mail being sent would still remain in the wordline is if the D-mail did not cause a divergence. That is, if the D-mail’s impact wasn’t strong enough to cause a worldline change (like with Daru’s D-mail trying to win the Rei-net game against Faris), nothing would happen and the fact that the D-mail was sent and received could be seen.
Tell me what you think! I'd really like to perfect these explanations so tell me if there's anything wrong or something that could be better clarified.