ubuntu - Postfix: Relay mails from specific domain

Question

I'm using postfix in combination with Zarafa as Groupware (local server) and import mails from an external server to a mailbox on my local server. I do this, because of restricted resources on the external server.

So incoming mails at the external server are redirected to my local server - but sending mails is done on the local server. The problem is, that the rx-Record of the domain of the external mail server does not point to my server - so my send mails are marked as spam, especially in GMail.

Therefore I want to relay the outgoing mails of the local mailbox to the external server to fix this issue. I want to restrict this relaying to this single domain - there are other domains on the local server witch don't need to be relayed, because the domains are owned by me and the MX record is correct.

Answer 1

Got it to work with relay server:

main.cf

sender_dependent_relayhost_maps = hash:/etc/postfix/relaymaps
smtp_sender_dependent_authentication = yes
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_type = cyrus
smtp_sasl_security_options = noanonymous

... and creating relaymaps and sasl_passwd file