can't send email

If AT&T provides you with a web-based email interface, you should try logging in using your favorite web browser to see if you can send emails. If you can, then you verified that it is not something amiss with your account.

Otherwise, you may have adware/malware present on your Mac that could trigger these warnings when sending emails. I suggest that you run Malwarebytes for Mac to detect and remove any, if found. It comes as a free trial, and afterwards, you can elect to stop using it, use its "after-trial" free version, or purchase a subscription. The free trial should be all that you would need in this case.

I also recommend that you run an EtreCheck report and post it here. With this report we can help you analyze your Mac to see if anything else may be causing this issue. The report does not contain any personal information and is for diagnostics use only. The following user tip gives you all the details:

• Using EtreCheck to Troubleshoot Potential… - Apple Community

Who is your email provider? That is, is it Apple's iCloud, Microsoft's Hotmail/Outlook/Live, Google, or someone else? Are you trying to send email messages with the Apple Mail app on your MacBook Air? Which exact model is your notebook and which version of macOS is it running?

I set up an email with a lot of names and must not have done something right. It is frozen or something and I cannot delete it. It keeps coming up when I go to mail but closes when I try to delete it. Help

Are you running any third party apps that claim to protect? Like an anti virus app.