Office 365 and Workflow Cloud are different architectures and have different owners.
From the perspective of your question, Office 365 is managed by Microsoft, even not by your tenant admin, and Workflow Cloud is managed by Nintex.
I think that for Office 365 you could use Exchange 2016. Consider that SharePoint Online supports outgoing emails for Email Alerts & Workflow Notifications. Outgoing email and SMTP servers are pre-configured for each tenant and they can’t be managed. In that case I'd contact the helpdesk on your Office365 tenant.
About Workflow Cloud I have no idea how to do it, and suggest you to write an email to support@nintex.com
Keep us updated about any information you get about this subject.
Hi,
thanks for your reply. I understand they're different architectures. The fact that SharePoint Online supports outgoing emails for Email Alerts & Workflow Notifications is good point. We will look into that soon and evaluate the Nintex trial. I will try to report back.
Thanks.