AOL email error 521 generally occurs when there is a problem with the reverse DNS lookup. You can refer to the solutions given below to fix the error so that you can continue sending emails via AOL:
• Step 1: Repair the Windows registry related to error 521
• Step 2: Scan the PC for any malware that may have caused the AOL error
• Step 3: Delete the temporary files and update the driver for your computer
• Step 4: Uninstall the program and then install AOL once again
If you need additional assistance to implement these solutions you can call the AOL customer care number and seek help from a certified AOL expert.
Read more: AOL email customer support | AOL email customer service | AOL email technical support