This is very common issue; I've personally helped a company after they lost much more than this, and had to help prove to insurance/govt agencies/etc. Turn on DKIM, DMARC, and SPF records for your mail domain. Also, never send invoices over email that contain any payment terms (eg: accounts, addresses to mail check to, etc) they should always be in some sort of protected portal. Tell every customer never to accept payment term details from you over email, phone, etc. If you or your client has insurance, start documenting every part of your case with screenshots into a file, and document everything you know NOW, including timestamps, etc.<p>EDIT: Also, I'd suggest taking orders via a secured portal, and also autheticating large orders by calling a number for a client you already have (never trust their website, or an email from them). Unfortunately, you're out of luck that money.