Why burner domains fail on X
X maintains aggressive suppression lists for known disposable domains. Sign up with an address from a free-mail cluster and you can hit instant rejection, an "unable to complete your request" error, or a shadowbanned account that none of your posts reach anyone.
A genuine temp Gmail bypasses those lists entirely. gmail.com is the gold standard of email infrastructure, so deliverability is near-perfect and the platform has no disposable-domain heuristic to trip.