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 larger than 5,000 addresses will require a dedicated hosting solution from us. Note: Dividing one large list into smaller lists to get below this limit is not allowed. You cannot send 12,000 emails per day (500 x 24 hours) on a shared account.