Why not use the hosting that provides mail function in the first place?

That's the one i was trying to clarify all along, it is like this. Many uses freehosting + a paid domain name these days. But most of these *free* hosting are not upto standard.
ex: assume that host1 is a very reliable, ad free hosting service but no mail() enabled
and host2 is a free service with all features but NOT reliable and even has ads
In a condition like this the user would like to have a combination of both, which is unlikely to be given free. As a foundation to make this combination MAIL4EVERY1 is there. You can use host1 as your host while using the mail() of host2. Hope it is clarified.

How do you connect to second server? Do you use curl or anything like that?

It is done using an AJAX request as the use of sockets would be disabled as well

Why not use smtp instead of mail function?

Even smtp needs the underling mail() function that is for php. So even if you use a relayer or a script like phpMailer it wont work as the mail() disabled

Is this not termed as spamming? Also does your mail come into inbox or goes into spam folder?

NO, this is not spamming but abusing this service may lead to spamming as well [That i'm trying to make a solution of]. The delivery of the mail relaye COMPLETELY on the way of host2(as of the previous example). The one i showed here will deliver it to the inbox. But there are certain free ones that deliver it to spam as well. For this i will be developing the use of smtp relayer like google from the other end (host2) for reliable mail delivery

How do you handle the blacklisting of such free hosts?

This is another issue when relating to the script. but the blacklisting will be done with/without the script as anyone can abuse the mail() function if it is enabled. But as for its effect on the MAIL4EVERY1 script assure you that it would be minimal as you can easily change your mailing host (host2) from the script so that if one is blacklisted another can be used. AND i am trying to develop a paid server in the future (if i got enough $$$ ) to make a free server available for the users of the script. If one i managed to provide this kind of a paid hoting for free ;
ex: user1 buys paid hosting with mail() and make it available for another 100 users to use
like that it can be developed but spamming will be a HUGE issue

AND thank alot shabbir for asking these questions as it will remove the doubts of other and i was just unable to explain the system well untill you asked those
thankx again