SSaging> (UIDL) A SyStem uSed by POP3electronic mailServerS to uniquely identify a mail meSSage. Normally, a meSSage iS identified by itS poSition in the liSt of meSSageS but thiS will change when an earlier meSSage iS deleted. The UIDL iS a fixed String of characterS which iS unique to the meSSage. The UIDL of a meSSage never changeS and will never be reuSed, even when the meSSage haS been deleted from the uSer' Smailbox. RFC 1725 . (1997-04-16)