+@item :leave
+Non-@code{nil} if the mail is to be left on the @acronym{POP} server
+after fetching. Only the built-in @code{pop3-movemail} program (the
+default) supports this keyword.
+
+If this is a number, leave mails on the server for this many days since
+you first checked new mails. In that case, mails once fetched will
+never be fetched again by the @acronym{UIDL} control. If this is
+@code{nil} (the default), mails will be deleted on the server right
+after fetching. If this is neither @code{nil} nor a number, all mails
+will be left on the server, and you will end up getting the same mails
+again and again.
+
+@vindex pop3-uidl-file
+The @code{pop3-uidl-file} variable specifies the file to which the
+@acronym{UIDL} data are locally stored. The default value is
+@file{~/.pop3-uidl}.
+
+Note that @acronym{POP} servers maintain no state information between
+sessions, so what the client believes is there and what is actually
+there may not match up. If they do not, then you may get duplicate
+mails or the whole thing can fall apart and leave you with a corrupt
+mailbox.
+