There is a 500 hourly email limit per domain. This limit is also applied towards mailman. If you send over this amount in any hour most of the emails will bounce back with an undeliverable error.

Many of our servers have a 60 pop checks per hour limit per email address. If you go over this you're likely to get a wrong password error message saying login incorrect. Just wait an hour and it will automatically unlock 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. Any mailing list larger than 5,000 will require a semi-dedicated or dedicated hosting solution from us. Note: dividing one large list into smaller lists to get below this limit is not allowed.

Mailing Lists Rules
  1. Anytime you're sending a message, no matter how large your email 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 which can be found in your control panel under fantastico. If you do not throttle and you try sending, let's say, 500 emails, the server will try sending all 500 in 1 second, which is not possible. This will cause the server load to go very high and for the entire server to be sluggish until this process is completed. It is our job to keep the server 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 crashing the server.
  2. Any mailing list over 900 emails is only allowed to be sent 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 Mon-Fri.
  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 justify it. This is spam and will result in termination of the offending account.
  4. Any unsolicited email being sent will result in termination of the offending account. We take a zero tolerance stance against sending of unsolicited email. 
  5. No Direct SMTP mailers are allowed. An example of this is DarkMailer. Any mail should be relayed through the local MTA (localhost:587) for further delivery by the server and not done directly by scripts.