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?

I know you can be put on a whitelist with each of those companies. I'm not sure how you'd go about that though. My best recommendation would be to send the invitations in small batches and then waiting a few minutes before sending more. What timeout to use, I dunno, and if you're getting any measurable traffic, you may end up getting queued til next year, so that may not be the best option. If you can think of any existing web site that does this that you could email the web master, that may be best. Of course you could always try a mass mailing and the blacklist you get put on will let you know how to get back off ;)

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).

I worked at a community web2.0 site and ran into this issue. What I wiould do to be safe is to call AOL/Yahoo/MSN/GMAIL etc... and find out what is there limit. This will save you the heartache of getting blacklisted. http://linuxdynasty.org "Where IT pros share their knowledge"