DOH! I was too hasty in posting! As soon as I made my comment I refreshed the QuickBooks page and the red exclamation mark of doom reappeared!! Although it has sent me a copy as I’m cc’d in. Also I was able to send two other invoices after that to separate customers and they worked fine. For what it’s worth, I seem to remember having trouble with invoices bouncing back from this customer before, through a different invoicing platform. Something to do with a firewall or spam filter their end if I recall correctly…