Removing the "Additional Mailbox" using the Advanced Outlook settings some times doesn't unlink extra mailbox from the Outlook profile. To manually remove it, you need to use the ADSI Edit tool.
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- Run adsiedit.msc on the server and connect to the Default naming.
If you get a "Packet needs to be fragmented but DF set" response, lower the packet size in 8, 10 or.