In SharePoint on-premises (SP 2013) I'm using Rest-API to send an email notification to the external user on a button click but I'm getting this error
"The e-mail message cannot be sent. Make sure the e-mail has a valid recipient."
for cross checking purpose I tried with SP Designer list workflow but no success in it as well,but if I share any site to any external user im getting a notification for same that means the server is configured to send an email notification to the external users
please let me know if anyone has some solution to it