I've got the steps to fix this email credentials issue in QuickBooks Desktop (QBDT), Whymeplease.

You can streamline your bookkeeping tasks by directly connecting your online email services to QuickBooks Desktop. The program makes it easy to send transactions to your vendors and customers in a timely and efficient manner.

However, if you encounter a message about an incorrect email or password while trying to connect your email, there's no need to worry. We can perform some procedures to help you rectify the problem.

First, let's update your QuickBooks to the latest release so you always get our security patches. Then email a test transaction to yourself.

If the issue persist, follow these steps to review your email settings and filter preferences:

1. Enable access for less secure apps.
2. Set access preferences for specific apps like QuickBooks.
3. Go back to the program and send a test transaction again.
4. If you still can't send emails, you can turn off the 2-Step Verification feature for your Gmail account.

However, if the error persists, follow the last series of steps in this article to learn how to check your antivirus settings and allow port exceptions: Fix webmail password issues in QuickBooks Desktop.

I'm also adding these links to learn how to customize your invoices and resolve email server connection error:

• Fix Error: Could not connect to the email server
• Use and customize form templates 

If you require further assistance concerning incorrect credentials, Gmail configurations, or any other tasks related to QuickBooks, please feel free to contact me here. Wishing you a pleasant day ahead.