You receive this error message when another eMail client already tries to fetch eMails for this account. 

In case the Internet connection (DSL/ISDN/Modem) terminates during the retrieval or the forced disconnect is executed usually a security file remains (said .pop file). This prevents your mail box from data corruption. Should this be the case the protection will be removed automatically after a few couple of hours.

Please wait for 24 hours before you contact the support. After 24 hours at the latest the protection should be removed.

Should the problem continue to exist contact our customer support via eMail with the following information: 

Customer ID
Order / contract number
Involved POP3 box
Error message you are receiving from the mail server