We are using Zimbra Open Source edition on three of our servers. Two of them are only for relaying, so they forward all email to the primary one.
On the primary Zimbra Server the "User-specified forwarding addresses" function is not working, when the mail is received through the relay servers.
When the mail is directly delivered to the primary Server, everything works fine.
When the mail is delivered through one of the secondary Servers, the mail is delivered to the account, but it is not forwarded, as it should be.
Both of the relay servers IP addresses are set as MTA Trusted Networks on the primary one.

In the logs we can't find anything about this, which could lead us to the solution.

Can anyone help?