SimpleLogin is an email alias service that lets you generate unlimited alias email addresses for when you sign up to services, newsletters, etc. It's almost entirely similar to Apple's Hide My Email except SimpleLogin is an open source implementation (so you know exactly what's going on with your data), not tied to your Apple ID, and finally owned by a company with a long track record for protecting privacy: Proton.
Most people might have an email address like
jim@wikipedia.com that they use all over the web for everything. The problem is many people that get your address abuse it by passing it off to spammers or if a service gets hacked bad actors can search the database for your email address and know that account belongs to you.
Email aliases let you generate unlimited random addresses like
orange.pufferfish12@simplelogin.com that forward any mail sent there to your personal inbox, effectively acting as a buffer between your personal email address and a service/newsletter that requires your email. Additionally you can reply to that alias address and it gets forwarded to whoever emailed you in the first place. You can also use your own custom domain name for your aliases like
my.cool.alias@yourdomain.com. You can generate a unique email for everything you sign up to and that way you'll know exactly who leaked your email to a spammer (e.g, if a spammer emails orange.pufferfish12 and I set that alias up for my Amazon account then I know it was Amazon who leaked it). The bonus is you can delete the alias at any time to prevent anyone with that alias from reaching your inbox ever again.
Proton Mask (or Proton Alias, they're still deciding on the name) is what SimpleLogin will be rebranded to soon when it officially joins the Proton suite of tools. It will expand its capabilities from simple email aliasing to also include:
1) Credit/Debit Card Aliases. Basically email aliases for when you need to pay for something online. This is huge and I personally cannot wait for this to get built. Every time you need to give your card number to an online merchant you can generate a unique card number just for that site which you can disable at any time. Want to buy something from a site but you think the card number might be saved and leaked later? Just generate a virtual card number and delete it right after the purchase.
2) Phone Number Alias. Same deal, burner phone numbers that forward calls to your real number.
I think Proton Mask/Alias will be one of their best products once fully realized. They're at the beginning stages of development right now so the card number and phone number features are a little ways off.