If you have access to the set domain DNS records, I highly recommend using a service like SMTP2Go.
It is a sort of SMTP proxy and is trusted by most carriers and delivery is much more reliable than Gmail.
This allows companies that use Microsoft 365, GMail (Free and Work) to also prevent using the less secure authentication (SMTP/POP). Unique username and password for each account if you prefer. We use it for all of our clients, with each having a budget of emails allowed per hour, day, etc.
Works with VZW, ATT, Tmo, etc.