The Starplex forum

A user who is using Outlook 2013 and wants connections to two Exchange mailboxes on the same server.

The result he is getting is that e-mails are stuck in the outbox. This is true of both mailboxes. The only "solution" I have found thus far is to remove one of the mailboxes from the profile and have them access the one removed via webmail.

This is not a solution, rather a workaround. The problem seems to be commonplace... A fix is not.

This is something to check...

Found here > http://www.reddit.com/r/sysadmin/comments/1dwhfm/exchange_2013_cached_mode_additional_mailbox/

We fixed this issue by doing the following:
You need to make sure that the shared mailbox is "auto mapped" and that the correct AD permissions are set.
We found that the Exchange 2013 Powershell and the Exchange 2013 GUI were not setting the correct permissions in AD.
Remove all delegate permissions from the mailbox you are granting access to
In Outlook remove any additional mailboxes from the delegates profile
Set the mailbox permissions either via the GUI or via Powershell (Add-MailboxPermission -Identity "John Smith" -User TomJones -AccessRights FullAccess -InheritanceType All) Auto-mapping is enabled by default. If you disabled auto-mapping at any stage, you will have to enable it again (Add-MailboxPermission -Identity "John Smith" -User TomJones -AccessRights FullAccess -InheritanceType All -AutoMapping $true)
Then check the Security permissions on the AD account of the User that you are granting the delegate access to
Check that the delegate has: Send As, Read account restrictions, Read Exchange information, Read Exchange Personal Information (anything that is already enabled should remain enabled)
Also check the AttributeEditor settings on the AD account of the User that you are granting the delegate access to and make sure the delegate is listed under the MsExchDelegateListLink attribute