External Email Address Limitation for Mass Email in Salesforce

I am Sakthivel Madesh, a certified Salesforce Advanced Developer & Administrator working on Salesforce Technology since 2011. I am currently working in Techforce Services as Sr. Salesforce Developer. I have worked on multiple technologies Like PHP, Java, Salesforce, I am very passionate about Salesforce and to earn 15x Salesforce Certification. I love Trailhead for learning Salesforce Skill and Become a Trailhead 5x Ranger too. I Love to Learn & Share my Salesforce knowledge to Salesforce Learners/Beginner using My Blog and Salesforce Community.