Total message count in the mailbox.

Number of messages with Recent flag set.

UIDNEXT: The next unique identifier value. The next unique identifier value is the predicted value that will be assigned to a new message in the mailbox. Unless the unique identifier validity (UIDValidity) also changes , the next unique identifier value MUST NOT change unless new messages are added to the mailbox; and the next unique identifier value MUST change whenever new messages are added to the mailbox, even if those new messages are subsequently deleted. The next unique identifier value is intended to provide a means for a client to determine whether any messages have been delivered to the mailbox since the previous time it checked this value.

UIDVALIDITY: The unique identifier validity value. Any change of unique identifiers between sessions MUST be detectable using the change of UIDVALIDITY. If unique identifiers from an earlier session fail to persist in this session, the UIDVALIDITY value MUST be greater than the one used in the earlier session.

Number of messages with Unseen flag set.