This should be the answer. As soon as the user enters a valid email (regex test) send a request to server to figure out what path they need to go down in the “federated flow”. What we shouldn’t do is diminish the experience for some because the flow for some others is different.

You shouldn't have a giant sign asking people for something they don't have, it's confusing and discourages people from using your product.

It should be onblur, otherwise you'd send requests for:

- me@example.c

- me@example.co

- me@example.co.

- me@example.co.u

- me@example.co.uk

And you'd have to account for all those tricky race conditions happening there

And me@example.co could be the email of another user, so you'll never be able to login with yours.

HelpScout's login is anothrt good example! Definitely echoing other's thoughts that it's the correct way to handle it