It is possible. You need to play with form settings:
General-> After the form is submitted -> redirect to URL and
Email notification. They both work with macros and you can access form fields in macro. The only thing is you can't send email or not based on some condition. Form will always send notification email, but you can put a macro in Recipient emails:
Where("PostCode='" + PostalCode + "'").Count > 0? : UserEmail: "SomeCustomAddressNotInUse@company.com" %}
UserEmail and PostalCode are fields in the submitted form. All form data submitted by user are available on 'redirect to URL' page, so you can use macros there to show your message etc.
After form submission the information is always recorded in table Form table, regardless weather postal code is good or bad.