Hi Laurent,
maybe my explaination wasn't clear enough.
Let us say the recipient is "thecry@gmx.de" (this email address doesn't exist but the gmx.de is working).
On my server are only service email addresses:
- service@domain-1.tld
- service@domain-2.tld
- service@domain-3.tld
Now my server should decide, by sending domain, which relay server should be use to send at the email address thecry@gmx.de.
If i send with "service@domain-1.tld" the relay server "mail03.domain-1.tld" should be use.
If i send with "service@domain-3.tld" the relay server "mail10.domain-3.tld" should be use.
My postfix decide's correct but the relay server said "said: 550 #5.1.0 Address rejected".
Then i send an email with "service@domain-1.tld" to "system-administrator@domain-1.tld" (this email exists in the company not on my server) and postfix us the relay server "mail03.domain-1.tld". But in this case the relay server accepts my email and deliver the email to system-administrator@domain-1.tld.