How to route mail in Postifix based on recipient address

If you do processing based on RCPT TO address, you are going to flood this person with spam, because it will disable any further spam checks.

Your only option is to use check_sender_access.

smtpd_recipient_restrictions =
            check_client_access hash:/etc/postfix/access_sender
            reject_unauth_destination,
            reject_rbl_client b.barracudacentral.org,
            reject_rbl_client cbl.abuseat.org,
            reject_rbl_client bl.mailspike.net,
            check_policy_service unix:postgrey/socket

Like so:

fromuser@domain.com      OK
domain.com               OK
fromuser@                OK

dont forget to postmap access_sender after you create it.

Disclaimer: this answer won't work as intended when process email which had multiple recipients with both internal and external domain in it

With postfix multi instance, the solution would be trivial. Just setup transport_maps to second instance, then do rewriting in second instance.

Without multiple instance, you need two smptd processes, two cleanup processes and access map.The idea is do filtering in access maps instead in transport_maps. When an email has external domain recipient, it transported to second smtpd daemon. Then second smtpd daemon would call second cleanup daemon. First cleanup daemon won't have canonical maps parameter, while in second cleanup daemon, we will add that parameter to do rewriting.

In main.cf, define check_recipient_access in smtpd_*_restriction. For example:

smtpd_recipient_restriction =
   ... 
   check_recipient_access hash:/etc/postfix/external-filter
   ... 

In external-filter file define a filtering mechanism

external.example.com    FILTER smtp:[127.0.0.1]:12525

In this example we will setup second smtpd daemon listen in 127.0.0.1 port 12525.

Now, setup second smtpd and cleanup daemon in master.cf

[127.0.0.1]:12525  ....  smtpd -o cleanup_service_name=cleanup_rewrite

cleanup_rewrite .... cleanup -o canonical_maps=hash:/etc/postfix/mysender_rewriting

File mysender_rewriting would contains a sender mapping as you intended.