+@item :leave
+Non-@code{nil} if the mail is to be left on the @acronym{POP} server
+after fetching. Mails once fetched will never be fetched again by the
+@acronym{UIDL} control. Only the built-in @code{pop3-movemail} program
+(the default) supports this keyword.
+
+If this is neither @code{nil} nor a number, all mails will be left on
+the server. If this is a number, leave mails on the server for this
+many days since you first checked new mails. If this is @code{nil}
+(the default), mails will be deleted on the server right after fetching.
+
+@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.
+