This came up in a case and the cause wasn’t immediately obvious. The customer used MAPI to open a number of mailboxes using OpenMsgStore. They found that if they tried to multithread their code, they’d open a few mailboxes, then they’d start getting MAPI_E_FAILONEPROVIDER. The odd thing was the same code running against Exchange 2007 did not experience this problem.
It turns out what they were running in to was Exchange 2010’s new client throttling feature. The purpose of this feature is to prevent a single client from causing a Denial of Service on the Exchange server by throwing commands at it and dominating it’s time. You may find that for certain applications, the default throttling policy isn’t appropriate. You can build a new policy using New-ThrottlingPolicy. For MAPI applications (whether they use Outlook or Exchange’s MAPI), the settings starting with RCA (RPC Client Access) are the ones you want to set, particularly RCAMaxConcurrency. Once you’ve created a policy, you can apply it to a user using Set-Mailbox.
An important note: In the policy, you’ll see a CPUStartPercent setting. This setting only applies to EWS, as confirmed by David Sterling in his comment to this post.