There are tools to make email guessing much easier. That is, without sending an email, you can check the address validity. Example:

https://verifalia.com/validate-email

However, not all mail services are configured to respond usefully to this sort of "is this address valid in your domain" type enquiry. I've just checked a few sample email addresses with a local provider and it appears to work there.
(for the technically minded: SMTP VRFY and similar)

Any decent provider will have disabled SMTP VRFY on external mail gateways, it serves no useful purpose but acts as a weakness for email guessing and therefore spam sending.