Why am I getting an "ERROR: Could not complete request." message when using the webmail login?

The rest of the message will typically let us know what is going on.

Reason Given: EXAMINE failed: Unable to extend mailbox: Disk quota exceeded

This is typically caused by a users mailbox that has used up all of its allocated disk space. This is caused typically by one of two reasons. First, the user's email client is not configured to remove messages from the server. Second, the user has messages from the WebMail system stored on the server. Keep in mind, any folders (including Sent and Trash) that contain messages are consuming part of the disk space allocated for your email account.