Skiff says they encrypt email subjects while protonmail does not. I cannot determine from browsing if they encrypt the sender and receiver addresses.

Skiff's free tier also has a 'generous' free tier with 4 aliases, 10GB of drive (only? combined with email?) storage, and custom domain support (? !)

Hello! I'm one of the founders. We do end-to-end encrypt email subjects. Sender/receiver addresses are not to ensure email goes to the right person. 10 GB drive is used across all Skiff products.

And domain support?

I've been using my personal domain on Skiff and been loving it personally! Simple to setup (nothing different to the usual) and it all works seamlessly!

Just that, it's one inbox in the Free tier. Since I'm using on my own domain, I prefer having one inbox for all addresses!

Yes, we have custom domain support with DKIM/DMARC/SPF all enabled.

I use FastMail but would consider moving. Can I have unlimited aliases when using my own domain? I use their MaskedEmail function to generate aliases for each service and have > 150 at this point.

Yes, unlimited aliases on any domain

you can transfer a custom domain on the free tier. we also let you register custom domains directly inside the product.