Error "The message could not be delivered to one or more recipients" for a valid and active email address in Office 365

Issue

When using "Send an Email" action with a user in Office 365, the following error is occurred : 
The message could not be delivered to one or more recipients

Resolution

1. Make sure that the email is for a valid and active user. 
2. Terminate and re-run the workflow.
3. If the issue still persist, contact Nintex Support with the details of the email address, this will expedite the discovery of logs. 

Additional Information

• The issue is intermittent and only occurs for periods of time. 
• The root cause is the email address has been soft blacklist by Nintex Mail Service, due to it previously attempting to send an email with an attachment that exceeded the size limit.