iPhone and iPod touch: 'Mailbox Locked', account is in use on another device, or prompt to re-enter POP3 password
Symptoms
When accessing a POP3 email account from iPhone or iPod touch and another computer or device at the same time it is possible for one device to be locked out, which can result in either an authentication error or "Mailbox Locked" message. If you have installed iPhone OS 3.1 or later you may get this message:
If you have a version of iPhone OS earlier than 3.1, the text of the message may vary depending on your email provider, and can include "POP Locked" or "POP busy". Some email providers also limit how often an email account may be accessed to once every ten minutes or longer and give a similar message.
If you are not receiving messages you expect on a POP3 server, and have received those message on another mail client, please see this article.
Products Affected
iPhone, iPod touch
Resolution
The POP3 mail protocol was designed, and is implemented in many cases, to support one connection at a time. If your email provider is not configured to allow multiple connections, when two different devices attempt to access the mailbox at the same time, both devices may get locked out, or you will be prompted to reenter the password.
Some steps you can take to prevent this from occurring are: not leaving your computer's mail application open, and increasing the interval to fetch mail on your computer and iPhone or iPod touch. This setting is available on the device by choosing Settings > Fetch New Data.
If the POP3 mail account is locked out, quit the mail applications on all other devices and return to the Home screen on iPhone or iPod touch. Once all email accounts are no longer accessing the POP3 account, wait for the server to reset the connection. This can take five minutes or longer before you will be able to retry. To make sure your iPhone or iPod touch is not keeping the connection open, try turning off your iPhone or iPod touch.
If your email provider is limiting how often you may access your email account, set the device to check manually or configure Fetch New Data to check less frequently.