I just had this for the first time today on one of my invoices. Hopefully just a one off, but I have two comments to make.

1) It would be nice to be able to view the sending log so that I can see which email address it failed to - this invoice has been sent to two different addresses, and BCCd to my own address. When resending, I have no idea who to send it to.

2) It would be useful if we could clear the 'Needs Attention' error for this invoice. I have now ensured that my customer has received a copy of the invoice. It no longer needs attention, but this error persists.