Members of shared mailboxes or delegates with ‘Send As’ rights to user mailboxes are unable to send emails This issues occurs when using an autocomplete entry that already resides in the Outlook Client auto-complete cache.
Non-Delivery Report – The email address you entered couldn’t be found When attempting to send an email to an existing address stored in your email auto-complete selection list, or a new email address typed in manually, a non-delivery report is received stating that 'The email address you entered couldn't be found.
