If you're using Yahoo! Mail, follow these steps instead:
Let's review your security settings by going to your Yahoo Account Security page.
Turn on the Allow apps that use less secure sign in and Two-step verification features.
Once done, go back to QuickBooks and email yourself a test transaction.
If you still can't email invoices, use the Yahoo's generate third-party app passwords tool. This will create a random password you can use to connect QuickBooks. Then, email yourself a test transaction in QuickBooks. In the Password field box, enter the third-party app password. Then select OK.
If your using other email services, you can check your security and filter settings, then make exceptions or rules so QuickBooks isn't blocked.
Says my email password is invalid. Can't email invoices
I've got your back, @Nex-level.
To clarify, are you using QuickBooks Online? Also, are you using your personal or business Gmail address to send invoices?
For most general issues in QBO, clearing the browser's cache can do the trick. Browser caches help speed up basic browsing that allows you to save more time when loading repetitive images or information. However, it can impact processes and cause this unexpected behavior when your browser accumulates too much historical data. Not to worry, you can clear them in just a few easy steps.
In the meantime, we can start by accessing your account through an incognito window to rule out the possibility of a webpage issue. You can refer to these shortcut keys to open an incognito window in all supported browsers:
If it's responsive, I recommend that you clear it's cache. By clearing the browser's cache, you can remove that historical data and access QuickBooks with a clean slate.
I've added here some articles to learn more about sending invoices from your Gmail address: