We have some Exchange Online rules that add a big red warning at the top of e-mails that come from external sources, urging people to be more cautious and that they may be phishing attempts. It would be helpful if we would have the ability to add a custom e-mail header to e-mails sent from the "Assign a task" or "Start a task process" action, at a minimum when using Lazy Approval (which should give Nintex more control over the e-mails).

Adding such a custom header like "X-My-Custom-Header: <GUID" would allow us to add exceptions to the rule in Exchange for emails we generate with Nintex, reducing the changes of the e-mail mistakenly being considered a phishing email due to the warning we add to emails from external sources and people not acting on the e-mail in time.

There is an option to add a custom e-mail header using the SendEmail Utility REST endpoint and we've done that for a custom SharePoint-hosted add-in to remove the warning from e-mails we generate. See this Stack Exchange Thread... https://sharepoint.stackexchange.com/questions/150833/sp-utilities-utility-sendemail-with-additional-headers-javascript