I appreciate the additional information you're sharing with us, Julius.
There are a couple of reasons why an email from QuickBooks is not being sent to customers. This includes browser issues, company settings, and servers or domains that are not configured to accept or send emails from QBO.
You mentioned that the issue is only happening in a specific client's file. Most probably, it has something to do with the email address in the settings section since other accounts were able to send transactions successfully. I'll share some steps on how to resolve issues with undeliverable emails in QBO.
If the client has recently imported QuickBooks Desktop data into a QuickBooks Online company, you may need to reset your email address. Here's how:
- Go to Gear or Settings ⚙ icon and select Account and settings.
- Proceed to the Company tab, then select the edit icon in the Contact info section.
- Review each email address and make changes as needed.
- Click Save, then select Done.
If you're still getting the same result, clear and then re-enter your email address in the same window. Then, send yourself a test transaction without clearing the body of the email.
Here's an article for more details about the steps that I've shared: 3 Solutions For You When Customers Aren't Receiving Your Emails.
Please share updates on this thread if you're able to send invoices without any issues. We'd want to make sure everything is sorted out.
