[lug] Unlock .username.pop lock

I have a problem with some locking on my users e-mail accounts

some error comes up upon send/receive on the Outlook client.
ie /var/spool/mail/.username.pop lock

i do i get rid of such lock, it seems this proble frequently occurs.
the particular e-amil account ois setup on one account , which rules
out the possibility that someone else will be down loading from the
same account.

i am running a sendmail on linix server

Could anyone assist in srting this problem.

Thank you

Owen