Sending mass-emails to 20K people at once (in theory)

Question

My database contains 20,000+ verified email-addresses. These are user's emails with permission (users set in options) to receive newsletters, updates, etc... from website. I actually never used their email addresses for any purpose but now I want users to get some updates from website periodically (once per month or two months). My wonder is more about how to send same email to 20K of users as fast as possible and what restrictions I might face doing this?

I have some ideas how to do this using Cron and PHP build-in mail() function but I can't send 20K emails at once (may I?). Also, I know that emails will most probably end up in junk/trash/spam folder at receiver's (that's fine) but not sure is there some law against sending mass-emails or restrictions (defined by hosting service).

What I need is answer how I can do it and what's best way to accomplish this task? This is not "give me solution" question, just need some tips because I've never sent more than 5 emails at once in my life and couldn't find answer to this question. Wonder how large services, such as Facebook, Twitter, Youtube inform their users (probably millions of them at once)?

Note: Sorry if this is not right place to ask this question, I had second thoughts about where to post it, here or at StackOverflow, but according to how my problem looks like (I don't provide or ask for code), guess it would be closed at SO as "not constructive" or even "off topic".

Answer 1

You will face a few problems when sending all that email at once. or one by one.

SPAM BLOCKERS

In order to send that amount of email you need to setup SPF, possibly some sort of certificate signing, use a server with a static ip, preferably with reverse dns. When you start getting even larger you have to contact some email postmasters direct to get put on a white list by submitting legitimate reasons and paperwork. We used to send about 250thousand emails in a day, sometimes 4 times a month to our subscribers.

We wrote our own programme- but there are many available.

Also you can just loop each email and send it one bye one.. then try to hyper thread it .. eventually breaking down into domains and maybe you can send them all in about 1 hours with some clever programming.

Good luck

Answer 2

Short answer: Do the static IP and reverse DNS. You can but don't send them in one hour. Mail servers don't like thousands of emails in a short while, you'll be blocked. I'd say even more if all that mail comes from a mail server that just popped in the net.

Long answer: I send 2.000 emails once a week. I use a program in php to send them, it's free software, I don't know if I can name it here so I won't. The web site is orange, it's in php to manage lists (newsletters). My server is a 1 GB ram xeon 2.4GHZ 4 cores with Debian, it sends 4 mails per second (around 100K each) with postfix (all outgoing mail is checked for viruses), so that should be 14.000 an hour. If you send too many emails to one domain, some of them might have some blocking feature. Yahoo had/has something like greylisting, I remember I had to contact them more than once for 3 months until they stopped the temporary rejects. My ISP rejects mails if it sees too many connections in a short while so in postfix I decremented the connections, I think from 20 to 10. By the way Gmail sends 5.000 mails per second, I heard it in a debconf (10,11,12) from one of the developers.