Unlock .username.pop lock

[Owiza]

Please could someone assist:

I have a frequent problem, where an error pop up :

/var/spool/mail/.username.pop lock,

upon Send/Receive on the outlook client

this problem frequently occurs. i have tried to increase the server
timeout time from the outlook client, and this is the
only computer on which the e-mail account is configured , therefore
rules out the possibility of concurrent access of
the account from different machines.

could someone assist in that respect.

Rgds

Owen