If any one of those properties change the UID value will be altered and the client will download the message again. MDaemon uses the filename (including extension), size, and timestamp to generate the UID values. The only time UID values will change is if the properties of the file change.
If a UID value for a message changes the client will download the message again. Thu 08:50:45: +OK POP3 Server signing off (2 messages left) Thu 08:50:45: Accepting POP3 connection from to The log should look something like the following: Open the POP3 log that includes the time frame when this occurred and compare the UID values returned by the UIDL command to see if they changed. You can find this option by going to Setup / Default Domain & Servers / Logging / Options. If you have the option enabled to Log multi-line protocol responses in MDaemon the log should show you if the UID values were changed. If the client forgets the UID value of messages that have been downloaded or if the server changes the UID values messages will be downloaded again. When using POP3 and leaving a copy of messages on the server the client keeps track of what has been downloaded by storing a UID value for each message that it has downloaded.