AOL Mail Error 521 5.21 is one of the common errors occurring while you work on it. No need to worry about this error on AOL Mail because you can remove this by carefully following a few methods. Clean all corrupted or damaged files, scan with File Checker all junk files, and download the Windows System. You can troubleshoot AOL Mail Error by calling AOL Helpline Number. One of the technical executives will attend your call and provide a proper solution to contact itself to remove this particular error.