public long UidValidity { get; }

Public ReadOnly Property UidValidity As Long
	Get

Property Value

If UIDs from an earlier session fail to persist in this session, the UidValidity value will be greater than the one used in the earlier session.

The client may save this property value for each folder and then, on each new session, compare saved value with the value obtained from the server. If UID validity value returned by the server in the current session is larger than saved value, the client cannot rely the UIDs of messages in the folder are the same as they were during the earlier session. However, mail servers which fail to persist UIDs between sessions are very uncommon nowadays.

Reference