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This happens if the Mail account lost connection with the server. We may need to delete the account and add it back. On the iPhone, go to Settings > Mail, Contacts, Calendars. You will see the bellsouth. net account there. Tap it and delete it. Then add it back by selecting Add Account > Other.
Enter the following server details if asked
Username : Your full email address
Password : your email password
Outgoing server : pop.att.yahoo.com
Incoming server : smtp.att.yahoo.com
Please let me know if that didn't work, so we can continue troubleshooting.