If you haven't got a chance to see the following guide, please check it at first:
https://docs.zen-cart.com/user/email...le-mail--gmail
We have gave up using gmail to send emails, but use aws ses instead. Our gmail account has 2-step verification, so smtp method is not working natively. The recommended way in this post is "allow less secure authentication" in gmail. However, this feature is retired by google already. If you don't have 2-step verification, this guide "may" still work. But 2-step verification is really necessary.
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