Gmail rejecting messages

Plugin is reaching gmail, but gmail is not allowing message to be sent.

The response was:
Message rejected. See https://support.google.com/mail/answer/69585 for more information.

Already opened a support.google.com thread: https://support.google.com/mail/thread/3358203?hl=pt-BR

But got no response up to present.

Could you please help us if plugin has any config we can change in order do help solving this issue?

Regards,

The page I need help with: [log in to see the link]