You have a requirement where if an Account/Individual Customer have more than one e-mail address maintained than the workflow rule should send e-mail to only to main e-mail id.
Reproducing the Issue
Go to Administrator work center.
Go to Workflow Rules view.
Create a workflow rule to send an e-mail to Account/Individual Customer.
When the workflow rule is triggered the e-mail is sent to all the e-mail addresses maintained in the Account/Individual Customer.
This is the expected system behavior and the email is sent out to all the email ids maintained for the recipient.
If you need this functionality urgently you may contact your implementation manager to help with such request or approach directly at SAP Cloud Service Center (firstname.lastname@example.org).
Note: The services of SAP Cloud Service Center will be charged as packaged services based on fixed prices.
An alternative option will be to check SAP Cloud for Customers Idea's Forum and submit your idea:
Workflow rules, e-mails , KBA , e-mail , workflow rules , LOD-CRM-WKF , Workflow , Problem