Hello, tramach. 

I know how inconvenient it would be to enter the email password every time you send an invoice. Don't worry, I'm here to help you get sorted out.  

To isolate this case, let's ensure you're using up to date QuickBooks desktop version. Then, check your security and filter settings. Make sure QuickBooks isn't blocked.

When you're done, you'll have to follow Steps 3 in this article: Fix webmail password issues in QuickBooks Desktop to troubleshoot the problem.

Once everything is fixed, you can start emailing your invoices. For additional reference, you can read this article about personalizing your invoice to beautify its templates: Use and customize form templates. 

I'll be here all ears if you need further help. Just hit back in this thread. Take care always!