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Try these steps so we can rule out any interference and we can go from there:
This error occurs because the mail server you are trying to connect to already has an open connection to it for your email account. This can occur because you are already connected via another device, or in a situation where the mail server has not correctly closed the lock at the end of a POP3 session.
Firstly, you should ensure that no other devices are trying to connect to your mail account. If you have a Blackberry, iPhone, or similar device, that downloads your email, or another computer that also accesses the same email account, then these should be disabled. Note that some people download email from one account to their Gmail, or other, email accounts via POP3. If you have configured an account to do this, then you should disable it to check if that is interfering with your connection.
If this does not sort out the problem, then the second thing to try is disabling your anti-virus and anti-spam software, or at least disabling the email scanning function if possible. This software can interfere with the connection and cause a delay which in turn causes the error as the connection is still open. In particular, this software can cause this problem when a malformed mail is downloaded. Note that this option should be done to diagnose the problem; it is not recommended to permanently disable anti-virus software. It is recommended to use an email server that has built in anti-virus checking to prevent malformed messages from reaching you.
Thirdly, if you still have a problem, there could be a malformed message on the server that is not coming through. Check using webmail (where possible) for any emails that have not been downloaded and delete any unwanted emails. Also check for emails with attachments and delete them if possible.