at can support the "disconnected" mode of operation, by providing
 unique-ids for messages in the folder. This interface is based on
 the IMAP model for supporting disconnected operation. <p>

 A Unique identifier (UID) is a positive long value, assigned to
 each message in a specific folder. Unique identifiers are assigned
 in a strictly <strong>ascending</strong> fashion in the mailbox.
 That is, as each message is added to the mailbox it is assigned a
 higher UID than the message(s) which were added prev