I'm planning to make my app passwordless and I've seen sites implement things differently when one puts in an email they haven't actually used for an account on the site before when trying to login.
On Medium, if I put in a new email I'll get the message "We just emailed a confirmation link to email@example.com. Click the link to complete your account set-up."
This makes the signup process fast, but I'm not sure if this message is strong enough to make it clear to some users that this was a new email being used. This could frustrate users who had forgotten which email was used for their existing account and would now take the time to realize they're using a new account before trying to login again with the right email. However, that scenario may not occur often enough that its worth designing the UX to prevent it.
Zeit does design things so if I don't have an account and I put in a new email I'll get the message "There is no ZEIT account associated with this email address. Continue with signup?" and I can click to signup.
This prevents one from realizing they used the wrong email, but it adds an extra step to signing up and I think it could confuse users who weren't paying attention to the fact that they clicked login rather than sign in.
What do you think is a better flow and why?