Overriding a method with Generic Parameters in Java?

No, it’s not overriding it properly. Overriding means you should be able to cope with any valid input to the base class. Consider what would happen if a client did this:

Monitor x = new EmailMonitor();
List<NonEmailAccount> nonEmailAccounts = ...;
x.performMonitor(nonEmailAccounts);

There’s nothing in there which should give a compile-time error given your description – but it’s clearly wrong.

It sounds to me like Monitor should be generic in the type of account it can monitor, so your EmailMonitor should extend Monitor<EmailAccount>. So:

public abstract class Monitor<T extends MonitorAccount>
{
    ...
    public abstract List<? extends T> performMonitor(
        List<? extends T> accounts);
}

public class EmailMonitor extends Monitor<EmailAccount>
{
    @Override
    public abstract List<? extends EmailAccount> performMonitor(
        List<? extends EmailAccount> accounts)
    {
        // Code goes here
    }
}

You might want to think carefully about the generics in the performMonitor call though – what’s the return value meant to signify?