I am hosting an alumni site that has a lot of registered users. i wonder if it will be treated as SPAM if i send emails to all 300 users, through the web server. And how about 1000, 5000 users?
you could send out an email to groups, asking permission to add them to such-and-such group. If you feel so bold and noble, you could set it up so that for them to okay being added, they have to click on a link in the email.
Something to think about. Why are you emailing them anyways? Wait, is this alumni site one you run, or just one someone else is running and you host it?
Best way to handle it is to make a Mailing LIst (preferably if its for your school alumni then call it the (such and such) Alumni Mailing List.
You want to start with Confirmed OPT_IN and to run a confirmed opt-in mailing list :
1. Set up a form to accept their email address to sign up for your list.
2. Your server will then send out an email message that says that someone from their IP address (captured from the sign up form) has signed up for your mailing list. If they are the correct person, they should reply to it, and your server will auto-add them to your Mailing list. If the address was inputed incorrectly, whoever received it should ignore the message and if your server doesn't get an answer (say in 24 hours from signup), it will dump the email address from the Database.
More info can be found at these links (read them, understand them and make sure you DO understand them). Double check with your HOSTER (both email and website) on their rules about Mailing lists: