Even major services. I tried to create an Esty account the other day, and it rejected all addresses from my Fastmail-managed personal domains, and would only take a gmail address.
Luckly it turns out you don't need an account to order, and I was able to use my custom domain for an unregistered checkout.
This must be related to the tld you're using, right? Not that Etsy only allows @gmail.com addresses?
I've noticed that some sites will specifically ban public domains from throwaway services like mailinator, but I can't recall having encountered a blanket tld ban.
I wasn't using a weird TLD. It was a .us that only my family uses.
I didn't try too hard to get it to work, or figure out exactly what their criteria were, but I kept getting blocked until I used my old gmail. Their customer support wasn't very helpful, either.
Even if Gmail is currently delivering the messages (for now...), other service providers have to manage spam in their own way, and TLDs are sometimes a really strong signal for message reputation. For example, 99% of email traffic from .info addresses is spam, and the other 1% is mostly spam too.