Office 365 Shared Mailbox prompting for Password

Hello,

We have setup a shared mailbox for one of our clients which works fine.

But one of the users has his mailbox setup at another organisation which has full access to this shared mailbox. The other organisation has Exchange Server 2010 hosted internally.

I've gone through this thread here but found no answer.  https :// community.office365.com/en-us/f/158/t/275312

(Company A) User1 xyz @ xyz.com > Full access to shared @ xyz.com (No password prompt)

(Company B) User1 xyz @ xyz.com > Outlook prompting for password for 'shared @ xyz .com' (Internal Hosted Exchange 2010

I've tried to enter in his Office 365 Details but still prompts for it.

Any ideas ? Thanks