I have over 4,000 users who subscribed to weekly updates via email. To send an email, I simply use:
mail($to,$subject,$message,$headers);
Which works perfectly fine. My question is: how many emails can you send at once? Can I send all 4,000 at once by stepping through array of emails inside a loop? Or will that cause a time-out situation, wjere only part of the list receives emails?
Generally, your limit is PHP's memory allocation and script timeout, both of which can be increased. A bigger limiting factor is spam firewalls, which typically limit the number of emails that can come from a server and to another domain. Bulk email services use intervals to limit the amount of mail sent to any given domain during a given time period.
PHP has no limit, and you can do what you say and run it in a tight loop.
On linux it is sendmail, or a sendmail clone. If it starts to get overloaded it should start queueing your mails, and delivering them when it can. For Windows you have a socket connection, and I suppose the mail server might start refusing connections if it got overloaded.
Unless you are in a hurry to send them all, consider a sleep(2) between each call to mail(). If all goes smoothly then try sleep(1) the next time. Then usleep(500000) next time, etc. (UPDATE: following Matt's suggestion, put set_time_limit(10) inside the loop to avoid running out of CPU time.)
If you want them sent out even quicker you round-robin with multiple local relays. (On windows you could handle this by setting the SMTP php.ini setting; on linux you'd use on of the libraries that sends email over a socket.) Or consider a professional mail broadcasting service, where they take care of all these issues for you.
EDIT Just saw your comment about running it from your Mac. I'd assumed a server situation, with a real mail server with a global IP. If your Mac is behind a firewall you are most likely set to forward all your email to your ISPs mail server. Your ISP may well get upset about receiving 4000 emails from you in a short period of time. (E.g. they may assume your machine has been compromised by a virus and shut you down.) Also, on shared hosting, agreeing not to send out lots of email is often part of the contract. So be careful there too.
Considering that loop will be sequential you can send them all at once by looping. It's more mailserver dependent, rather than PHP.
Although if you're using WAMP, that might cause a problem, because behavior of mail() under windows is different. Instead of talking to an MTA it tries to use sockets to deliver mail.
