Supposing that the system could read what is after the @ in the mail field at the form and at the end of it direct you to that mail client (gmail for example)... is it worth it, is it just a stupid thing? I hope i made my self clear, my english is not good at all.
If you are talking about redirecting the user to their email client so they can verify their account I would not do this. You may think it is being helpful, and for some it may be, but for many it will be annoying.
For example, I never go to my email in my browser when registering anymore. When a site says its sending me an activation link or code I just wait for it to pop up on my phone and activate it there so that I don't lose context in my browser.
Another issue is people can set up their own email domains that may not exactly link to a website under the same domain. Or people may use email forwarding to send their mail between accounts to check elsewhere. In other words you can't be sure you're redirecting the user to the correct place so don't redirect them at all.
And finally, to make the reference I do in many answers, Shneiderman's "Eight Golden Rules of Interface Design"
Support internal locus of control. Experienced operators strongly desire the sense that they are in charge of the system and that the system responds to their actions. Design the system to make users the initiators of actions rather than the responders.
Users will not be expecting to be redirected away from the site when registering for it, therefore they will no longer feel as if they are in control of the system. This will likely lead to them not trying to return to the site once redirected away.
EDIT: After rereading it I don't think you are automatically redirecting just adding a link for the user to follow if they so choose. This makes it less annoying than I made it sound but for all the reasons I listed still unnecessary and could cause many issues.