Since this year (2019) Google will have more strict policy regarding the Gmail scope. If in the past for sending notifications we were used to request the Send Email scope to user, now it is less comprehensible and on Google side you will have to provided lot of more details and your application can be rejected. So it become time for thinking to send email as the application and not as the user. Say Hello to RelayApp service
Source: Build your own Mail Service for Apps Script with Glitch & Gmail SMTP relay
