I. Cannot be enabled unless a BCC map

Failed to save enabled features : Automatic BCCing of outgoing email cannot be enabled unless a BCC map is defined.

Solution

1. Go to Webmin → Servers → Postfix Mail Server.
2. Click on the BCC Mapping icon, and make sure the Sender BCC mapping lookup tables field is set. If it is not, you should enter a map specification like hash:/etc/postfix/bcc . Then click the Save and Apply button.
3. Go to System Settings → Module Config, and change the Allow automatic BCCing of outgoing email? field to Yes, then click Save. If Virtualmin prompts you to re-check the configuration, do so.

II. Mail with virtual host

1. Need to install Postfix
2. Enable feature from Virtualmin > System Settings > Features & Plugin select Mail for domain > Save
3. Enable feature on domain Virtualmin > select Domain > Edit Virtual Server > Enable Features > select Mail for domain > Save

III. Reject by destination and networks

NOQUEUE: reject: RCPT from mail-il1-f179.google.com[209.85.166.179]: 454 4.7.1 <your-email@hostname>: Relay access denied; from=<sender@gmail.com> to=<linh@daphalestudios.com> proto=ESMTP helo=

setup mynetworks that include your local domain

add smtpd restriction

smtpd_recipient_restrictions = permit_mynetworks defer_unauth_destination
smtpd_relay_restrictions = permit_mynetworks defer_unauth_destination

IV. Recipient address rejected: User unknown in virtual alias table

re-create user again

V. Reject by unknown destination

NOQUEUE: reject: RCPT from unknown[your-ip]: 454 4.7.1 Relay access denied;

Add defer_unauth_destination into smtpd_relay_restrictions