What OS are working on.(Windows Or Mac)
Are you getting any error message while sending the email from the Lightroom. Let us know the error message word by word.
Also, check if you have configured the email account correctly in Lightroom, you can refer the link below for the same.
Related Link: Email photos
Thanks for your reply but it was not the solution to my issue. I'm using Google's 2-step verification and it was necessary to enter an app-specific password supplied by Google. Here's the link -Sign in using App Passwords - Accounts Help. The "correct solution" you refer to will not work if you're using 2-step verification.
Please check the link below for 2-step Verification for Google.
Also, try and disable any active firewall on the system.
Let us know if it helps.