As title states my php mail function is NOT working with Gmail. However, it does work when sending to other domains such as hotmail and yahoo. I have checked my spam box.

Any ideas. I have noticed that when I use phpmailer.php and link it to my gmail account as the SMPT server I have no issues sending emails.

Not sure if this is related but there are transmission rates, if you send more than 150/200 mails in 15 minutes to the same domain you can get blacklisted.

Each mail provider is different for limits and sensibility.

A part that, if you're sending mails through Gmail SMTP you're limited to 500 mails per day, where: each recipient counts one email and each email can have no more than 100 recipients. So if you send five emails with 100 recipients each, you hit the daily limits.