SMTP Error: 454 4.7.0 Too many login attempts, please try again later

I’ve managed to change the hostname to use smtp-relay.gmail.com, however, I am getting this error when I am sending many welcome emails after each other due to importing of 100+ students.

This article might assist in rectifying this issue using Pooled SMTP:
https://stackoverflow.com/questions/40550149/smtp-error-454-4-7-0-too-many-login-attempts-please-try-again-later#answer-54299705

I look forward to your input.