Force relay for only one mailaddress, all others are local

Hello,

i want to use my kopano for the domain xx.y. I receive all mails via fetchmail and send over relay.
Local hosted accounts: test1@xx.y, test2@xx.y
User test1@xx.y receives also another@zz.z via fetchmail.
test1 has to send with “another@zz.z” over smarthost.
This all works perfect.

Problem is when sending to mailaddress (external@zz.z)
I can not send mails from my server to external@zz.z
only another@zz.z is local, all others @zz.z is external

/etc/postfix/transport:
external@zz.z   smtp:[my.relay.zz.z}:587

/etc/postfix/main.cf:
myhostname = kopano.xx.y
mydomain = xx.y
transport_maps = hash:/etc/postfix/transport
mydestination = $myhostname, localhost.$mydomain, localhost


postmap /etc/postfix/transport
systemctl restart postfix

postmap -q "external@zz.z" hash:/etc/postfix/transport
smtp:[my.relay.zz.z]:587

Error:

kopano postfix/smtpd[6361]: NOQUEUE: reject: RCPT from localhost[127.0.0.1]: 550 5.1.1 <external@zz.z>: Recipient address rejected: User unknown in virtual mailbox table; from=<another@zz.z> to=<external@zz.z> proto=ESMTP helo=<kopano.xx.y>
kopano kopano-spooler[6375]: RCPT line gave SMTP error 550 5.1.1 <external@zz.z>: Recipient address rejected: User unknown in virtual mailbox table. (no retry)

Server wants to deliver the external@zz.z local but shot forward to smarthost

Can somebody help?

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