Hi,
Same issue happened for me. So I checked mail logs and it was saying "Relay access denied". Then I checked Postfix configuration and got to know that it was getting relay_domains from 'domain' table of postfix mysql database. Upon checking entries of this table I found the table is empty.
So this is an issue. Whenever I add a user or add a domain, the domain should automatically be inserted in this table for mail to work properly. But this did not happen. So I added a mail account on the domain and then the same domain got inserted to that table. Then I deleted the mail account for that domain. Now my email alias / forwarder works fine.
Conclusion: So after creating a new user or adding a new domain, if you need to add forwarder, you must first create an email account on the same domain and then delete it, then you add the forwarder, it'll work smoothly.
Dear Admins, kindly fix the issue in next CWP release.
Thank you.