A bug exists in OS X Mail where receiving email from the server through the POP protocol stops without warning. The problem is often easy to identify and fix. Here are the steps…
OS X occasionally chokes on a malformed email message when trying to retrieve it using POP. The messages will be left on the server (and can be verified using a webmail service) but Mail just will not grab them.
In my experience this is usually an email message that is 0 bytes long.
Removing the malformed email message will allow Mail to start working correctly again.
1. Open OS X Mail
2. ctrl-click (right-click)on your inbox
3. Select Get Account Info
4. A list of messages on the server will be created. Blue dots will show the unread messages on the server. If a malformed email is causing the problem, it is likely it is the first unread message listed before you read messages. As shown in the example below, it will often have a size of 0 as well. Select the message and click Remove From Server.
5. Confirm removing the message on the server.