Anyone able to advise?
I have solved this issue. It was due to permission of the sharedmailbox.
I could never get this to work. However you can use the normal send email and put your shared mailbox name in the "To" field.
As long as you have access to the shared mailbox, this works flawlessly.
Thanks Paul, I have checked with the client and awaiting a response. Is there a way I could test to find out what type the account it?
The error suggests you need to check the email address of the shared mailbox you are using. Are you sure the address is for a shared mailbox and not just for a regular mailbox that has been shared with someone? They aren't the same thing.