I'm using JavaMail 1.4.5 to access a pop3 server. The server I'm accessing does support the pop3 UIDL command but the method getUID does return null: http://javamail.kenai.com/nonav/javadocs/com/sun/mail/pop3/POP3Folder.html#getUID%28javax.mail.Message%29
On other servers it works fine.
I've narrowed the problem to the following: If the server does not list UIDL capability in the response to CAPA before authentication, POP3Folder will fail to retreive the message IDs. My test server does list the UIDL capability but only after a successful login but POP3Folder will not list the UIDLs. According to the RFC there is no harm in listing different capabilities in TRANSACTION and AUTHORIZATION state http://www.ietf.org/rfc/rfc2449.txt