Important places to troubleshoot & submit the requests

Troubleshooting for senders with email delivery issues

https://support.google.com/mail/troubleshooter/2696779?visit_id=638131370158694120-2634727032&rd=3#MXToolbox

Sender Contact Form

https://support.google.com/mail/contact/gmail_bulk_sender_escalation

Google Post Master Tool

https://postmaster.google.com/u/0/managedomains