Smarthost and same internal/external domain name and users

Local server and remote server with same domain name.

Smarthost configured to use remote SMTP with a user named the same as a local one (i.e. both serves have user@domain.tld). Mail server was not installed.

After installing nethserver-roundcubemail for internal use, e-mail notifications to user@domain.tld are delivered locally (not through the smarthost, not delivered to remote server).

To be able to deliver through smarthost changed destination mail for notifications for an e-mail address not matching a local user.

Is there a better setup allowing same local/external e-mail addresses being able to deliver externally?
Even if dovecot and postfix were to be restricted to green LAN?


Technically, what you want (I’ve done that too, for clients!) really violates a few RFCs on SMTP and DNS. Nevertheless, still doable!

A few tips to get it going:

Internal and External DNS must be separate (two different MX needed) = Split DNS.
Internal can resolve almost all external DNS records, the external one not!

Basic Magic to do the trick: a subdomain for mail…
eg: is your domain.
Internal main Domain name would be something like

Users on the external server would be:

Joe Miller (

Users on the internal would be:

Joe Miller (

Internal users also get a mail alias same as the external user-mail.
Mail arriving at the external Postbox gets forwarded (Or use POP3 pickup) to the internal Mailbox.
Smarthost is the external server, using IP on the internal server, and a dedicated user like (on the external server).

YMMV, but this should work as required.


Thanks for the reply.
Actually all e-mails are handled by the mail hosting provider.
Internal server was already setup as server.domain.tld resulting in mails having user@domain.tld, matching external domain. I could add a new mail (sub)domain to NethServer but don’t think I could change the current one. I don’t plan to replace existing hosted e-mail with NethServer.

Background info:
At the moment access to the internal mail is only required as a searchable archived copy of external one. Didn’t want to lose messages but needed to free-up space on one of the mail hosting providers. As I didn’t had time to properly test e-mail archiving applications (i.e. mailpiler) and the request was kind of urgent, used imapsync to get all messages from the hosting provider to a local Maildir through IMAP, just for one mailbox:

Host1 Nb folders:               15 folders
Host1 Nb messages:          231074 messages
Host1 Total size:      90249533534 bytes (84.051 GiB)