ATT Email creates a few issues sometimes and such as Sign in problems, server settings, etc. But not sending messages is also a new issue for everyone and it can resolve easily. Just follow my guidelines and fix it.

• Clear Cache and Cookies
• Update Your Browser
• Test signing in to your webmail using another browser
• Disable browser tools or add-ons
• Disable your firewall
• Enable Adobe Flash Player
• Make sure Java is up-to-date

Resource URL: https://emailspedia.com/cannot-send-emails-from-my-att-mail/