D-Mails aren't absolute. Remember, Daru send a D-Mail to send to himself the position of the cards during his Rail-Net match against Faris. He still lose. Okabe send the numbers for a loto but doesn't manage to win the cash prize he aimed. In other words, even if he tries that he had a great chance it would be pointless because of attractor field convergence. The death of Mayuri appears quickly unavoidable, no matter how much time Okabe have before and the strategy he adopts.

So wouldn't it be the same with Kurisu. In what you said even sending D-mail like in alpha world line it wouldn't change divergence number. But it did when Okabe send D-mail "kurisu is dead".

The concept of attractor field convergence imply there is a point where the worldlines diverge, and the divergence point can be only some events... and one of the events that change the world is the first D-Mail okabe accidentally send. All other D-Mail change worldline but never change the final fate of the world, in other words all the worldlines go to the next convergence point before diverge again.

Suzuha give some convergence point: The fall of USSR and the 2000 year crash.

The first D-mail, the one about Kurisu's death, did not change the events because of its contents! What matters is that SERN's spying network (ECHELON) detected a mail that came from the future; this intel made them win the race for the first time machine and so create a dystopia. As a result of this, Suzuha travels back from 2036 to 2010 with an incomplete time machine, destroying the Radio Hall, cancelling Dr. Nakabachi's conference, preventing Kurisu's death.