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I'm developing a social networking site "contacts inviter" script, which would send invitations to hundreds of contacts imported from GMail, Hotmail, Yahoo etc. I'm developing this script in PHP, and this script will eventually run on a Linux server. My concern is about the mail() function that would send email the invites. Lets say a user has 500 contacts (worst case scenario!) on their list, and they email the invite to these users, at one time. Since I'd be sending these emails out by my mail() function, wouldn't these hundreds of emails sent from my hosting server, to GMail, Hotmail etc., be considered as spam? What I'm trying to determine is the number of emails that can be sent out at once, before they're considered as spam. Any thoughts?

If you expect them to have more than 10-20 contacts you should definitely implement some sort of queue and send email slowly (and mix domains so don't send to ten yahoo ones in a row is possible but scatter them among others). Various sites have various rules to reduce spam so you will have to experiment carefully. It also depends on using a smart host (ie your ISP's mail relay).