smtp external mailtransport for internal mail

[trooper_]

Hi,
I'm new to Zimbra.
We've got domainA.com at mail@domainB.com
We have the domainA.com in zimbra and at mail.domainB.com with a few accounts foo@domainA.com and bar@domainA.com
To receive mails we use settings -> accounts -> add external account
We configured the mailtransport to smtp:mail.domainB.com:25
Sending emails from foo@domainA.com to bar@domainA.com are only delivered intern without errors, but we want to deliver mails to mail.domainB.com
Please don't asky why, we want it.
We can't use relay-mta because we are using several domains on the zimbra server. Some are only on zimbra and some are external primary.
Setting name of "smtp-host for incoming mails" to mail.domainB.com does no effect. setting mailtransport to smtp/lmtp port 25/7025 does also no effect.

Please help us.