My DAG has failed over, but Outlook clients won't connect

My company have two exchange 2010 server(Mail1 & Mail2) with DAG configuration. One day my Mail1 server down and the Mail2 auto take over! My question is now the end user can't access the mail2 using outlook .Even though i manual change the Microsoft Exchange Server to Mail2 in outlook ,but it will auto be changed back to Mail1(but this server is down now).Now the end user only can use OWA to access the Mail. help me! thanks!

It sounds like your CAS server doesn't know the database has failed over and is still pointing users to the failed server. Since you say you only have the 2 Exchange servers, I'm guessing it's 2 Exchange servers with the Mailbox, Hub Transport and CAS roles installed on them.

If this is the case, you need a hardware load balancer to ensure high availability of your CAS servers because you can't have both a DAG and Windows NLB (for a CAS Array) on the same box (the hardware load balancer effectively does what Windows NLB does, and probably does it better to be honest, being a hardware appliance).

My guess is if you run the cmdlet Get-MailboxDatabase |fl Identity, RpcClientAccessServer it will show mail1 as the RPC Client Access Server for the affected mailbox database.

If that's the case, issue the command Set-MailboxDatabase "<<Affected Database>>" -RpcClientAccessServer mail2.yourdomain.com and with a bit of luck, that will solve the immediate problem and your Outlook clients will connect back up.

Once you've done that, read the documentation on CAS Arrays, set one up and the set the RpcClientAccessServer property of the Mailbox Database to that of your newly created CAS Array. This will ensure that in the future all database failovers are almost seamless (just give Outlook a few seconds to reconnect).