I switched from desktop to online. I tried to email a customers their invoices a notification popped up saying "QB'S is unable to send your emails."

Hello there. I understand how time-consuming it must be to experience issues when sending your customer's invoices an email here in QuickBooks Online (QBO). Allow me to provide additional information regarding this concern and route you to the appropriate support. 

As per checking, I've found an ongoing investigation (INV-103794) related to the issue you are facing regarding the error message when sending an email. There is no specific timeline as to when will this be resolved. However, rest assured that our dedicated Product Engineers are actively working to address this matter and find a resolution as quickly as possible.

In the meantime, I suggest contacting our support team and letting them know about your experience. By doing so, you'll be added to the affected users list and will receive an email update regarding this ongoing investigation.

Here's how:

1. Log in to your QBO company.
2. Select Help (?).
3. Click Contact Us and choose a way to connect with us:

• Start a chat with a support expert.
• Get a callback from the next available expert.

Moreover, I've added this article as your reference to learn how to personalize and add specific info to your sales forms: Customize invoices, estimates, and sales receipts in QuickBooks Online.

Please click the Reply button should you need additional assistance managing invoices in QuickBooks Online. I'll be here to assist, 24/7. Stay safe.