よくある問題Error: This character has mail or has a heirloom item in their inventory, bank, or void storage and cannot be deleted
You must empty the mailbox of a character before you can delete it. Opening the mailbox only is not enough: it must be cleared.
A mail notification icon may not be present in the minimap if you have opened the mailbox but you did not take the items from it. Even if you see no mail notification in the minimap, make sure to visit the closest mailbox and empty it before logging off.