Recently we have been seeing a number of issues with Exchange Server using all the transaction logs that are available to a storage group. This results in Exchange store databases dismounting without warning.
Exchange uses transaction log files that are numbered sequentially. The first file name in the transaction log file sequence is "E nn 00001.log." The file names increase sequentially to "E nn ffff0.log." The maximum number of transaction log files that can be generated in a single sequence is 1,048,560 (0xFFFF0).
To prevent this issue, you need to monitor the log sequence numbers for the Storage Group. To reset the log sequence, the store must be shutdown cleanly and all log files must be deleted. For more information, please refer to Microsoft Knowledge Base article http://support.microsoft.com/kb/830408/en