i apologise in advance for my bad English, i hope you can understand me, but above i hope you can help me

I'm not an ufficial mta and the mail send to my domain are deliver in my isp server first, and every 15 min i fetch the mail from this server to my local zimbra server.

I have defined a sender_bcc rule, in postfix that when someone from my domain send a mail, a bcc are sended to anothe user called _user. For example
From: me@mydomain.com
To: other@otherdomain.com
Bcc _me@mydomain.com

the _user is like a backup user, however the _users are not defined in my isp server, in this server is only defined user1, user2, user3, but not _user1, _user2, _user3.

When i normally send an e-mail to another user in my doman there are no problem, but the e-mail are deliver locally, and everithing works so fine.

i'd like to find a method for deliver the email from local user to the isp server, and i think that the high priority is the best method.

For implement this i do a header_check in postfix conf directory like this:
/^X-Priority: (1|2)/ FILTER smtp:myispserver.mydomani.com

but my isp server respond that the _user not exist, and that is right, because this user not exist.

Id like to create something like this:

from: me@mydomain.com
to: other@mydomain.com
bcc: _me@mydomain.com

me -> other@mydomain.com ---> zimbra
\ /
-> _me@mydomain.com

from: me@mydomain.com
to: other@mydomain.com
bcc: _me@mydomain.com
priority: high

me -> _me@mydomain.com -> zimbra
othe@mydomain.com -> myispsmtp

Help me please.

Thanks in advance