In order to enable proper sharing across mailboxes, you should use a mailbox of recipient type 'Sharedmailbox'. Your administrator ...

1. In order to enable proper sharing across mailboxes, you should use a mailbox of recipient type 'Sharedmailbox'. Your administrator can create a shared mailbox by following the instructions from {0}.{1}2. If you really want to use a mailbox of recipient type 'Usermailbox' for sharing purposes, you might want to check if that mailbox has the appropriate Office 365 license. Your shared mailbox has invalid license assigned.