GMail SMTP rejects email from my domain, claiming it is a “security risk” if I haven’t authenticated

This evening I sent an email to a Cornell University address of someone, following a website interaction; I’d not written him before. I got this in reply from MAILER-DAEMON@cornell.edu (anonymized):

<firstname.lastname@gmail.com> (expanded from
    <some-id-here@alr.mail.cornell.edu>): host
    gmail-smtp-in.l.google.com[142.251.16.26] said: 550-5.7.26 This mail is
    unauthenticated, which poses a security risk to the 550-5.7.26 sender and
    Gmail users, and has been blocked. The sender must 550-5.7.26 authenticate
    with at least one of SPF or DKIM. For this message, 550-5.7.26 DKIM checks
    did not pass and SPF check for [gmx.com] did not pass 550-5.7.26 with ip:
    [128.253.150.158]. The sender should visit 550-5.7.26
    https://support.google.com/mail/answer/81126#authentication for 550 5.7.26
    instructions on setting up authentication.
    some-id-here - gsmtp (in reply to
    end of DATA command)

Plus, my domain (gmx.com) is the 10th largest email provider in the world (as of 2022, according to Yahoo Finance). It doesn’t make sense for Google to reject emails from it. PLus, I obviously can’t setup SPF or DKIM for a domain I don’t own.

So, what are people supposed to do? Tens of millions each need to setup their own SMTP service with their own personal domain, then setup SPF or DKIM for it?… Grrr.