Well, technically, you can't. But here's someone who can contact him for you, his management.

Marat Safin

c/o Elite Management
31 Princesse Grace Avenue
98000 Monte Carlo
Monaco

Okay, so I'll be quoting from Annie of TGFR to clear things up a little bit about contacting him:


"Contacting Marat Safin.... the impossible dream 

Dispelling the myth.......
Marat is not the easiest tennis player for a fan to get hold of - even at a tournament! He's an intensely private man, despite his oncourt showmanship and that's why a lot of people become so frustrated - on court, he looks so accessible, so friendly! And he is - but it's on his terms, not the fans.

If you are contacting him or his management to ask any of the following, don't bother:
1) Why doesn't Marat chat online with his fans?
Answer: It's not his favourite thing in the world. He loves his fans, but in his rare freetime, there are other things he would rather do. Internet chats are organised by his management

2) Why doesn't Marat keep an online journal/blog like Federer etc?
Answer: Too much like hard work  It's never going to happen, just accept it. Why would a private man journal his personal thoughts for the whole world to read, and for journalists to misquote? Nope, forget it.

3)Where can I email him/contact him direct?
Answer: *Ahem*, outside of a tournament basically you can't contact him DIRECT, however

Starting from 2005 the long-awaited OFFICIAL Marat Safin website has been launched at www.maratsafin.com (YAY!!!!!) where you can find all the latest OFFICIAL Marat news, updates, etc. and also get various opportunities to participate in competitions to win tickets to Marat's matches, tennis gear signed by Marat and other precious gifts once you have become a member of an official Marat Safin fan club 

There is also a forum on the website, where you can directly communicate with a Webmaster of the site, who is in touch with Marat and his management on a regular basis. 

Also you can write to Marat through his Management although replies are rare:

Marat Safin
c/o Elite Management
31 Princesse Grace Avenue
98000 Monte Carlo
Monaco

A few things to remember:

• This is not a fan club and replies are rare, but all mail is given directly to Marat when he arrives in Monaco.
  Tip: Do not mail him food stuffs as it may be confiscated by any European Customs & Excise (not to mention, going stale and mouldy....*ewww*)"

from http://guyfromrussia.proboards.com/index.cgi?board=maratinfo&action=display&thread=754


Before I forget, I am not anyhow related to Marat's management, okay? :) Happy mailing!