emailed collection letters

Are collection letters via email only acceptable, or must they be accompanied by registered letter? I have had clients claim non receipt of both emailed invoices and collection letters? This is a matter of cost as well as much as a disinclination to stand in post office queue for half an hour in a busy day.

Thank you,

Patrick Cloonan.

Hi David,
Thanks for the tip, have found a company called DidTheyReadIt.com who certainly pass the Ronseal test - that is, do exactly what they say on the tin (or web address perhaps) - you send en email, attach DidTheyReadIt.com (the recipent is unaware of this) to their address, and when the email is opened they send you an email, and another when it is read (though how they can tell that I can only wonder. Basically, as 90% of my invoices are sent via email, and as all too often I get the excuse or reason if you want to be kind, which currently I don't, that "we never received your invoice". Whether this is only good for evidence against the client, but then I believe that the only argument against paying an invoice is that the creditor failed to fulfill the contract, and that one is not obliged to send endless statements. Correct?
Thanks again.