I've sent an invoice from QuickBooks as usual. This time the status shows "sending error" sent in same format as the last 20 invoices? Customer is on holiday.

I can help you resolve the error you're having when trying to send an invoice, @darrenkerr19-bti. 

You can resolve this error by either checking your company and customer-facing email address, checking your location’s email address, and invoice format, or checking your sales forms email option. 

To check your company and customer-facing email address:

1. Go to Settings and click on Account and Settings.
2. Click the Company tab.
3. Select Contact info, then check your Company email and Customer-facing email.

In case you use location tracking, verify the email address format of the location you used on the sales form. If not, then skip to the next section.

1. Go to Settings, select All lists, then Locations.
2. Find the location you used on the invoice.
3. In the Action column, select the dropdown, then Edit.
4. Check your email address in the field This location has a different email address for communicating with customers and making changes if needed.
5. Click on Save.
6. Try to resend your sales form.

Check your invoice format and make sure it doesn’t have special characters (! @ # $ % ^ & * ( ) _ - = + : ") on your Description, Attachment name, Message, Email address, and Custom form templates. 

For more troubleshooting steps, check out this article: Fix the Delivery Server Down error in QuickBooks Online. 

Once resolved and you've already received their payments, here's how to record invoice payments in QuickBooks Online. 

Let me know if you need further help sending an invoice. I'm always glad to help in any way I can. Have a great rest of the day!