SummaryWhen attempting to email a shared mailbox via an Office 365 Nintex Workflow the 'Send Email' action is unable to resolve the Shared Mailbox account.
Check to see if the email account can receive emails from a different source e.g. Gmail.
If the shared mailbox can receive email, then you can utilise the 'External Email & Attachment'
If the Shared Mailbox was setup and configured as an Office 365 Shared Mailbox then you should receive emails with both options.