“Those blue dots show over 3,500 Gmail customers having the booking confirmation email they asked for delayed by up to 12 hours.”
A booking company sent confirmations its own customers requested, and Gmail sat on them for up to 12 hours with a generic rate-limit error. SPF, DKIM and DMARC no longer prove anything because spammers set them up too. So a new server’s reputation is whatever Google decides, and nobody at Google answers the phone. The complaint comes from Tutanota, a mail provider that also needs Gmail to accept its mail.