When the email is not deliverable should we consider that the email address doesn't exist?

Technically, no. There are several email providers who will set the account in an inactive state after a period of time which will cause the email to be considered as non-deliverable. However, the email providers may reserve the address in case the user ever wishes to activate it again.  In this case, the email address may be considered as non-deliverable but the registration itself can still exist.
It is also possible to have the same response for custom domains. The deliverable state is therefore dependent on the domain's settings. As a result, custom domains may fail to respond due to the SMTP MX record check, and their status will be shown as not deliverable.