"Unable to deliver..." message - any documentation?

I’ve received this warning message i nmy email approx 6 hours after sending an invoice:
"We were unable to deliver Invoice [seq number] to [Client Name].

Message delayed: The server could not temporarily deliver your message (ex: Message is delayed due to network troubles).

The Invoice Ninja Team"

Other invoices sent around the same time did not produce this message. Prior to this warning I got the usual “Invoice was sent” message.

My searches so far have not turned up any documentation on this matter.


1 Is this a warning of a temporary delay which will self correct?
2 If answer to 1 is “yes”, will I get a message informing me that it has now gone to the client?
3 If answer to 1 is “no”, then what do I need to do? (in order to get the invoice to the client? Hit “Email invoice” again? - that appears to be an active button when I view the invoice)
4 More generally, is this documented somewhere and what are good search terms to find it? (That’s a call to collaborate by the way if something needs writing).



  1. No, this won’t self-correct
  2. N/A
  3. If the email address is now flagged as bounced you’ll need to reactivate it, there will be an option when clicking ‘Email history’ on the client overview page.
  4. There’s some info about the codes here: Bounce API | Postmark Developer Documentation