UID is a String property that represents the Unique Identifier message attribute assigned to a message in mail-related protocols like IMAP4 and POP3. The content and format of UID is based on requirements in the mail protocol. 

For messages retrieved using TIdIMAP4, UID will contain the string representation of the persistent 32-bit integer value stored in mailbox data associated with the message envelope. The value in UID is assigned when a message instance is created and updated in the message collection used in TIdIMAP4.UIDRetrieveAllEnvelopes. 

For messages retrieved using TIdPOP3, UID may contain the arbitrary 70-byte value (maximum) which identifies a message in the POP3 maildrop. In POP3, individual byte values in UID must be in the range 0x21 to 0x7E. 

Messages marked as deleted in the POP3 maildrop are not returned with a UID value.

TIdIMAP4 

TIdPOP3