The sign went to press with the old phone number on it. The customer had asked for the change on the Tuesday, in a reply to a reply, under a thread called Re: Re: proof. Someone saw the first approval and missed the second message. A full sheet of vinyl and a day in the bin.
Printers and sign makers live on proofs. Each one is a version, each version gets a response, and the response is the only thing that matters when the job is on the bed.
The system watches the proofs you send. proof_v1.pdf goes out at 11:46 from your mailbox, and it logs who received it, which version, when. The customer's reply comes back and it reads it. A clear yes is recorded as an approval, with the version number and a timestamp. A reply with changes is logged as changes requested, with the changes copied out, and the job is marked as waiting for a new proof. A vague 'looks good but' is flagged for you to read yourself.
Nothing moves to the press list without an approval against the current version. If v3 is approved and you upload v4, the approval goes stale and the job drops out of the press list until v4 gets its own yes.
The record sits on the job. Who approved what, at what time, from which email address. When a customer says they never signed off on the spelling, you open the job and there it is.
This is the kind of thing we build. It runs in your own accounts, from the mailbox you already use for proofs. How many reprints did you do last year that came down to a missed message?