Originally posed Google Apps Developers Blog
Posted by Saurabh Gupta, Product Manager, Google Apps Script
There are two ways to send email in Apps Script: MailApp's sendEmail and GmailApp's sendEmail method. One of the differences between these two methods is that the MailApp’s sendEmail method doesn’t require the developer to be a Gmail user. For example, a Google Apps customer who doesn’t use Gmail, but uses Apps Script instead, can send emails through MailApp but not GmailApp.
Starting on September 13, 2016, users with free public Google Accounts (consumers) and Google Apps for Education and Google Apps Free edition users, will be required to have Gmail access to send messages through Apps Script’s Mail Service. Consumers can enable Gmail on their Google account after signing-in—note your Gmail will then become the primary address of your Google account. Administrators of Google Apps domains (Education and Free edition only) can use the Admin console to turn on Gmail for their domain.
This change does not require any updates to your code. You can continue to use MailApp as before; just make sure that you have signed up for Gmail. We realize that sometimes these changes are disruptive to our developers, but we can assure you that we put lot of care and deliberation into this process.