[Exim] .pop files

Hello,

this is probapbly a stupid question, but I am quite new to exim.
does anyone know what the .useraccount.pop-files are for that are
generated from time to time in the /var/spool/mail directory?
They seem to be created during the SMTP connection, and get deleted
again, but sometimes they stay. How do I have to handle them?

Thanks in advance,
Wolf-Guido Lutz