What is your mail policy and limits?
There is a 150 hourly smtp email limit per domain. If you send over this amount in any hour, any e-mails beyond the 150 limit will not be delivered and may bounce back with an error message.
Our servers also enforce a limit of 60 pop3 checks per hour per email address. If you go over this, you will be temporarily blocked from accessing the server your site is hosted on. Just wait an hour and it will automatically unblock you. To prevent this from happening again, make sure to disable auto checking or at least set it to something higher such as 10 minutes.
Mailing Lists Rules
1. Anytime you're sending a message no matter how large your e-mail list is you must throttle it. We recommend you throttle it to at the very least sending 1 email every 6 seconds. If the mailing list software you're using does not allow you to throttle, you must switch to something else. We recommend phpList. If you do not throttle, the server load will likely go very high and make your web site load slowly until the process is completed. It is our job to keep our servers up and running without being sluggish. Anyone who causes the servers load to go high will be suspended and the process will be terminated. If you choose not to throttle, you will most likely be suspended for resource abuse.
2. Any mailing list over 900 emails is only allowed to be sent to during off-peak times to prevent high server loads. Off peak times qualify as all day Saturday and Sunday, and 1am - 8 am eastern time M-F.
3. You are not allowed to mail to a mailing list you were given or purchased. This is spamming because they never agreed to you personally sending them mail. We do not care how you try to justify it, this is spam and will result in termination of the offending account.
4. Any unsolicited e-mail being sent will result in termination of the offending account. We take a zero tolerance stance against sending of unsolicited e-mail.