I believe I can create 5 email forwarders (free account), but when I try to add a second I get the message “this email alias already exists”. What does this mean?
This error means that you’ve already created an alias matching the one you’re trying to add. Say that you created an email forwarder such as support@mydomain.com. Because you’ve created this in the past, you’re no longer able to register that alias again.
You may want to check if a matching email exists, as this would be the issue. If it doesn’t exist and still throwing the error, please screenshot the error and I’ll notify developers. 
I hope this helps you! Let me know if you need more info 