AOL Error 521 is a technical error that is caused because of a problem with the Internet mail host. You should try rebooting your system and updating the internet browser to fix error 521. You can also use the Disk clean-up utility tool to scan your hard drive and clear unnecessary data. If you find that the error persists even after you implemented the solutions mentioned above, you can call the aol email technical support number and ash for help. Software experts will be able to give you advanced technical solutions to troubleshoot AOL Email Error 521.