When submitting an invoice or estimate the system will tell you that the email was invalid but only if you access it again directly. Can there be an automatic email sent if the item comes back as undeliverable? Or perhaps there can be some kind of identifier from the main page of the "quotes & estimates" and "sales invoices" screens.
Alert when invoice or quote emails are not delivered succesfully Merged
Notification 'icon' of email status for sent invoice on "sales" page Merged