Are you trying to send email alias security group using Send Email Action SP Online with help of designer workflow?
Well this is design behavior of SP Online and it doesn’t validate the AD Azure security group due to security enhancement and shows below error message while configuring in Send Email Action.
“The selected user(s) may not be valid on the site this workflow is published on. If a recipient is not a valid SharePoint user, he or she will not receive workflow emails.”
Also you can read:
– Configure Search Query Rule Office 365 SharePoint Online
– SharePoint Online Email enabled Libraries
– Create Azure App and Deploy to SharePoint Online as an App Step by Step
So let’s talk about solution approach.
1. Create SP group and add the email alias (Security group) to the group and configure the SP group in Send Email Action.
2. The functionality still works fine in SP 2010 workflow templates and you can use SP 2010 workflow for Send email functionality. Call SP 2010 workflow from SP 2013 workflow for Send email functionality.