Not able to send emails to the external users in SharePoint 2013- on premises

[anandrk4891]

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 he is 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

[#2]

The error message suggests that the receipent of the email to send is either not set or not properly set. Sending emails with the REST api is limited to valid SharePoint users for security reasons. You can't send to external addresses.

[#3]

Thanks Stephan for the reply, is there any alternate for rest api through which i can send email notification to the external users on button click (external users are getting site share notification ... i tried with designer list workflow to send notification but its sending mails to internal user but not for external) 

[#4]

Hi there,

I've had to go through this headache with external emails as well as attachments and whilst I can't give you our actual solution I'm sure I can give you some pointers.

Firstly head into portal.azure.com and go to 'Azure AD' and create a new application. From there you can set it use the 'Exchange API' and give it several permissions based on what your scope for the project is.

Here's a guide on adding and Azure AD App

Once that's done, go into the manifest of the AD App and change 'AllowImplicitFlow' to 'true'.

Now you have to set each Sharepoint web page that would be using this app in the Azure AD App - 'Reply URLS'

The next difficult part is considering security and protecting the Access Key exposed in an OAuth handshake. There's pages and pages on this stuff and I'd be wasting your time here. Once you've done the previous steps you can leverage whatever frameworks to authorize and protect the token.

JWT.io for example is a great standard.

And here's the REST API for Outlook/Exchange

Also here's a guide on authentication flows for Azure AD Apps

The main sticking points for me were the combination of different query parameter's required,

Here's what worked for me:

clientId = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx" (Azure AD App)

replyUrl = "https://tenant.sharepoint.com/reply.aspx" (You Decide)
endpointUrl = "https://outlook.office.com/api/v2.0/me/messages"
resource = "https://outlook.office.com/"

authServer = "https://login.windows.net/common/oauth2/authorize?"
responseType = "token"

url = authServer +
"response_type=" + responseType + "&" +
"client_id=" + clientId + "&" +
"resource=" + resource + "&" +
"redirect_uri=" + replyUrl

I have also found you can pass small strings in the "state" query parameter, it can be useful for certain functions in your SP solutions.

Hope this helps you out with your dev work.