Email custom sender gmail smtp

I am trying to configure a custom sender to work with my google workspace gmail. I have tried both smtp-relay.gmail.com and smtp.gmail.com. I have configured SMTP relay service in my google admin settings. I'm using port 587 for TLS.

I am getting error 421 for smtp-relay.gmail.com and error 534 for smtp.gmail.com. I'm not sure what these errors mean.

What could I be missing here?