I’m not sure in regards to Gmail, but Zoho does not work and outlook stopped working on an ecommerce site I was a part of administration on.
It’s been best practice to use a transactional email service such as Mailersend for quite a while now but growing more and more a requirement as time goes on.
Edit to clarify: it’s a block on the email provider side and not something that can be fixed on Discourse or server side. Most providers no longer support, if they ever even officially did at all, using SMTP to send transactional emails.