Introduction This error is thrown while sending email using apex if you are setting User Id in target recipient: setTargetObjectId(usr.Id) and also specifying target record id as: setWhatId(opp.Id). As per Salesforce documentation if you are passing record id in WhatId then you can only pass Contact Id in setTargetObjectId. Here is Salesforce article: https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_email_outbound_single.htm Workaround