Google can still automatically mark your message as spam if enough people mark it as such or you trigger spam identification too often (this can happen if you send frequent test emails with not much in them) - it’s not neccesarily an attack on you. But plenty of spammers will send perfectly well verified emails today, so you are just caught in the crossfire.

Try sending a gmail account you still have access to a couple of genuine looking mails (eg. At least a paragraph of legible text and with a signature), then marking the messages as not spam. That worked for me. Even after that though, one line messages can still trigger the spam detection.