Welcome Emailhelpneeded,
Thank you for using Apple Support Communities. Based on your post, you can't add your BellSouth email to the Mail app on your Apple products. We'll do our best to assist with this.
It sounds like you've removed and added back your email account, but that didn't work. We'd try to manually set it up using these steps: Add an email account to your iPhone, iPad, or iPod touch and/or Add an email account to Mail on your Mac You'll need the email details, use the Look Up website to figure those out: Mail Support
If it still doesn't work, it may require a secure key now: Get AT&T email on a wireless device
Set up or update AT&T email - Microsoft Outlook 2016
You may need to contact your email provider for additional support: Welcome to AT&T Support
Hoping this helps out, take care!