There is a 500 hourly outgoing email limit per domain on all shared servers.This limit is also applied towards mailing lists. If you send over this amount in any hour, the e-mails will fail and may bounce back with a undeliverable error.

Shared servers limit each connecting IP to 30 POP checks per hour. If you go over this, you will likely get a password error indicating your login is incorrect. To fix this, just wait an hour and it will automatically unlock. To prevent this from happening, disable automatic send/receive or set a higher time interval, such as checking once every 10 minutes.

Any mailing list over 900 email addresses 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 - 8AM Eastern Standard Time, Monday through Friday.

I get a bounce back indicating my email has been tagged as spam due to exceeding a 500 e-mail per hour limit.
You will need to wait until the next hour to continue sending emails.
If this problem does not go away by the next hour, please email [email protected]

CAN-SPAM rules
Online businesses that use e-mail marketing must follow all the CAN-SPAM rules.

• Identify your message as an advertisement.
• Include an unsubscribe mechanism.
• Include a valid physical postal address.
• Deceptive subject lines and false headers are outlawed.
• You will be held responsible if anyone sends emails on your behalf.