Office 365 and exchange 2010 mailbox issue

we are in a hybrid mode. a large number of users are still on premise (exchange 2010) while a few are in office 365

A few users had secondary mailboxes that they could open when they were on premise. These mailboxes were never removed from their outlook account prior to being migrated.

The users have been migrated the mailboxes they used to access are not


Outlook 2013 user in office 365 has mailbox access to on premise mailboxes. They can access the mailbox from within their mail box.

We want to remove those additional mailboxes since cache mode is turned on for office 365 users.

I tried removed it from outlook but it tells me to go into account settings and remove the mailbox. Outlook 2013 does not show any additional mailboxes in account settings. How can I remove these additional mailboxes?
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Vasil Michev (MVP)Commented:
Seems those were auto-mapped, so just remove the permissions via Remove-MailboxPermission or via the GUI. Or via ADSIEdit/attribute editor, the msExchDelegateListBL attribute.

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iamuserAuthor Commented:
I did remove the user's permissions to the on premise mailbox via GUI/EMS but the mailboxes in question still shows up in outlook 2013. User just can't open the mailbox which isn't what we want either.  We want to It remove the mailbox from showing up.
Vasil Michev (MVP)Commented:
Check the attributes, the GUI/cmdlet can fail in some cases, leaving the actual attributes in place. Another way to confirm it is to get the autodiscover.xml response for that user, the delegate will be listed on the bottom.
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