>>This problem is common and email, once the greatest thing since sliced bread, has become an unreliable way to send things like invoices, etc.
>>People used to say "The check is in the mail." Now they say "I never got the invoice." and they're often telling the truth.
>>Client tech people are fiddling around with alternatives, but nothing solid has surfaced.
>>Sounds like an opportunity.
>
>Impossible.
>
>The opportunity will be equally visible to advertisers. Whichever new channel of communication is opened, they will destroy just like they did with every other one.

Not always.
Teletype was spam free.
Fax killed that, but as you say advertisers eventually got those fax numbers and started spamming.