Don't send to Gmail over IPv6
The annoying thing about the major email services (Gmail, Outlook, et al) using reputation as a key input in their spam prevention algorithms is that they don't handle reputation in a way sympathetic to small and hobbyist mail server operators at all.<p>Google and Microsoft have the computation and data storage resources to forever record how much spam they've received from the mail servers operated by any given small company or hobbyist. If a domain's authoritative mail servers have a history of never sending spam, when that company or hobbyist (for any reason) has to switch hosting providers and gets a new IP address, Google/Microsoft should recognize that the domain, and its referenced mail servers, has never been malicious.<p>But instead, the majors treat a new IP address for a long-established domain as entirely new to the internet and assign it zero, or even negative, reputation.  And they don't care to address this because, well, why care about hobbyists and the rare small company insolent enough to try to self-host email?