It’s long been an issue with SPD2013 Workflows (yes, some people still use that today), that you cannot send an e-mail to an external email or a shared mailbox. SharePoint 2013 Workflow can only send emails to organization users and a shared mailbox is not a user.
If you try sending an email to an external or shared mailbox you will most likely end up with an error like this: “BadRequest to /_vti_bin/client.svc/sp.utilities.utility.SendEmail”.
So to do it, you have to get your SPD2013 Workflow to start a SPD2010 Workflow. the SPD2010 Workflows only task is to send that email to the shared mailbox.
SharePoint 2010 Site Workflow
Step 1: Create a SPD 2010 Site Workflow
Step 2: Create 3 Initiation form parameters
|emailbody||Multiple lines of text|
|recipient||Single line of text|
|subject||Single line of text|
Step 3: Create a “Send Email” action
Step 4: Input the parameters in the “To”, “Subject” and “Body” areas of the email
Step 5: Publish it
SharePoint 2013 Workflow
Now, the task is to create or edit your SPD2013 Workflow to start the SPD2010 Workflow with the correct parameters.
Step 1: Define 3 variables for use when starting the 2010 Workflow in the next step
Step 2: Create “Start a Site Workflow” action
Step 3: Select the “send email” workflow from the list
Step 4: Match the 3 parameters with the 3 variables
NOTE: The recipient parametervalue should be type = String.
You should end up with a result that looks like this:
Now, each time your SPD2013 Workflow runs, it will start the SPD2010 Workflow to send out the email. This is, so far, the only way I’ve found that a email to an external email or shared email can be sent from a SharePoint Designer Workflow.