The vast majority of the time the number one priority is reducing friction before a conversion. As much as a email confirmation prior to completion is more secure, the business case is far less strong.
Customers can fix their email later, they can contact customer support if they got something wrong.
Get them in the door ASAP, and either using the account, or complete an order. Don't redirect them to their email where there is a good chance they will either get distracted or the email will be delayed.
That's not to say you shouldn't also have your own measures in place to detect errors, or malicious checking if an email is associated with an account.
Customers can fix their email later, they can contact customer support if they got something wrong.
Get them in the door ASAP, and either using the account, or complete an order. Don't redirect them to their email where there is a good chance they will either get distracted or the email will be delayed.
That's not to say you shouldn't also have your own measures in place to detect errors, or malicious checking if an email is associated with an account.