If you get this Gmail error when trying to send an email to one or more recipients, and if you have tried deleting and adding those email addresses again does nothing but brings the error back up, the solution is very simple.
Save the email in the draft, You just have to log out, and log back in and try again.
In my case, I have changed my primary custom domain Gsuite email address to a different custom domain Gmail email address and I ran into this error. I just had to log out and use the new email address with the same old password, and that instantly fixed the issue.
Hari Swami
