I can provide the steps to resolve this email issue in QuickBooks Desktop.
If the email service doesn't work, its possible that the email settings is reset to default. Let's set up your Gmail under Secure Webmail. This ensures safer and secured connection between QuickBooks and your email provider.
Perform the steps below:
Click the Edit menu.
Select Send Forms.
In the My Preferences tab, click the Add button.
Choose Web mail.
Enter your Email ID and Email Provider.
Mark the Use Enhanced Security tick box, then click OK.
Sign in to your Intuit account.
Your Web mail provider's login page will display. Sign in and grant Intuit access.
Here's an article with the complete steps on how to set up your email service in QuickBooks Desktop.
Once done, try sending the email again and see if its working. If not, delete and recreate your email profile again in the Preferences to make sure you can send the forms this time.
Please let me know how this goes by clicking the Reply button below. I want to make sure you're back on track and I'm here to lend you a hand any time. Have a great day!