I have attempted to send multiple invoices putting email addresses comma separated in the "To" field but they fail to send to the recipients.

Hi there, @aaron-bg.

I’ll ensure you can send an invoice to multiple people in QuickBooks Online (QBO). This way, your customers can receive a copy of their transactions at any of their email accounts.

Since it works with the other email added when sending your invoices, check your junk or spam folders to see if the transactions were flagged and routed there. Once verified, clear the email section and re-enter your email address. I'll show you how:

1. Go to the Gear icon and select Account and Settings.
2. In the Company section, edit the Contact info.
   
3. Clear the Customer email field and re-enter the address.
4. Select Save and Done.

Additionally, you'll also have to clear the customer's email address to trigger the application and start afresh. Then, send yourself a test transaction, and if you still don't receive the email, try using a different email address with a different domain.

It's also best to configure your server to accept QuickBooks Online mail server hostnames and IP addresses to avoid QuickBooks emails being marked as spam.

Additionally, I've included these articles that will help you send reminders to your customers once their invoices are soon due. This way, you'll be able to ensure you're paid on time: 

• Send invoice reminders automatically or manually in QuickBooks Online
• Create recurring transactions in QuickBooks Online

Feel free to reply if you have questions about managing invoices, @aaron-bg. The Community is available 24/7 to assist you. Take care!