Then sign up for their postmaster tools and see what they're complaining about.
Then next use that same google account you needed for that to create a gmail address.
Then you setup something like CheckMK (or even just syslog on some random garbage VPS) that uses your own mailserver and sends towards your gmail address.
Then you mark the mails as "not-spam" and should they be rejected entirely open a ticket (annoying I know)