Are you saying your signup flow would automatically log the user in without confirming their email address (verify later)? I wouldn't suggest that for most websites as that would allow someone to signup with an email address they don't own.
For most websites transacting with potentially sensitive information, having an email sent to confirm you own the email address should already be part of the normal flow, so I'm not suggesting any extra step here. I was only suggesting an alternative email response in the situation you try to signup with an email that already exists. The normal happy path signup flow for a new account is not affected at all by my comment.
I think the sibling post by samwillis explains my view the clearest.
Basically, the business case for breaking the signup flow to require users to check their email is low. It interrupts flow and reduces conversion rates.
The suggestion then is yes, you are allowed to use emails you don't own to sign up for an account. The reason this is allowable is that who would want to do it? The account would be broken and the real owner of that account can pop your password.
For most websites transacting with potentially sensitive information, having an email sent to confirm you own the email address should already be part of the normal flow, so I'm not suggesting any extra step here. I was only suggesting an alternative email response in the situation you try to signup with an email that already exists. The normal happy path signup flow for a new account is not affected at all by my comment.