The Community has got you covered, @chris-dziubina.

I can provide a set of workarounds to isolate the issue, refer to the steps below:

1. First, turn on less secure apps and then unchecked Enhance Security.
2. Next, disable your firewall.
3. Then, reboot your computer.
4. Once done, attempt to send an invoice again. 

If the same result, you can attach a PDF format of the invoice to the email and send it:

1. Open the invoice and click the Print button.
2. Select Save as PDF and save in the preferred location.  
3. When done, open the email.
4. Attach the PDF version of the invoice to the email and send it. 

However, if the issue persists, I recommend contacting our technical support team to further investigate the root cause of the issue. Here's how:

1. Go to the Help icon.
2. Select QuickBooks Desktop Help.
3. Click Contact Us.
4. Give a brief description of your issue. Example: Unable to send an invoice by email.
5. Tick Let's talk. 

Also, you can always seek self-help articles on our Community page to help you finish your QuickBooks tasks in no time.

Feel free to get back here if you need further assistance in sending emails in QuickBooks. I'm always here to help. Stay safe.