最近公司邮箱储扩容，原有数据库没法满足现有需要，扩容后将所有用户邮箱全部都迁移到新的数据库上，迁移完成后十多天将老的数据库删除（系统自建的不能删），结果有部分用户登录出现500 错误。
检查CAS各项设置都没有问题，检查IIS日志发现登录不上的都会出现：DatabaseGuidNotFound 用户查的数据库GUID 是被我删除的那些数据库的GUID。
根据官方的最终解决方案需要打CU6,只能使用替代方式，删除所有客户端 cookie，然后重新并清除服务器缓存的MSExchangeOWAAppPool和MSExchangeSyncAppPool的应用程序池。
在微软官网发现这是EXCHANGE 2013 的一个BUG.
Users cannot access their mailbox in either Outlook Web App (OWA) or Microsoft Exchange ActiveSync (EAS) when both of the following conditions are true:
- All mailboxes are moved off a mailbox database.
- The original mailbox database is removed from the organization.
When users try to log in to OWA, they receive either a blank page or an "HTTP 500" error message. For example, they receive the following error message:
The website cannot display the page HTTP 500
Cause
This problem occurs because a user‘s mailbox database GUID (source) is contained within a client-side cookie and is added to the server cache on the Client Access server (CAS). The HTTP proxy on the CAS tries to locate the mailbox database by using the old database GUID within the cache. Because the old database GUID has been deleted, the attempt fails and returns a DatabaseGuidNotFound error.
Workaround
To avoid this problem, do not immediately remove the source mailbox database after the mailbox move is complete.
To work around this problem, delete all client cookies, and then restart the MSExchangeOWAAppPool and MSExchangeSyncAppPool application pools to clear the server cache.