event id : 9646 logged only for 1 user

Hello, 

MSexchangeis logged event ever day. 

Event id :  9646 (exchange 2010) 

Windows Event Message: Mapi session "e31793a0-a496-4767-811c-928199197261: /o=MLH/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn= xxxxxxx" exceeded the maximum of 500 objects of type "objtFolder".

This error is only for 1 user. 

I have searched on google. Found that this error can be caused by more number of subfolders in mailboxes. 

I have executed below command to check subfolders but no subfolders found in the mailbox. 

get-mailbox | select Displayname, @{n="FolderIdCount"; e={(get-mailboxfolderstatistics -id $_.Identity | select FolderId).count}} | Sort-Object -Property Displayname

What is the cause of this event? 

Do I need to increase threshold more than 500? only because 1 user is in problem. 

http://technet.microsoft.com/en-us/library/aa996193(EXCHG.80).aspx

Please suggest.