UPD: Wasn't SPF
https://collar.place/@Sominemo/112060082949309292
I've complained about not being able to receive verification codes from Twitter to get my data export earlier.
Uhhh.. it seems like having multiple domains in SPF record caused the issue for me? I'm not entirely sure about it yet though.
If it is indeed SPF — WTF? It's just for received email verification, why does it cause delivery problems TO my domain from Twitter?
But I tried switching to iCloud custom domain purely and it worked with it, then I switched back and it kept working until I edited the SPF record.
I'm rate limited right now to keep experimenting, though.