You are using incorrect url for SBCGlobal email login page.
Entered incorrect login details of SBCGlobal email account.
Forgot your password or someone hacked your SBCGlobal email account.
Your Internet connection is slow.
Wrong SBCGlobal email server settings.
Extensions of Web browser and plugins may be interfered.
Clear cookies and cache of web browser.
Above mentioned are the best reasons for SBCGlobal Email Login Issues.