なんですか？ / の後ろに i が抜けてるとでも言いたいんですか？
main.cf:
transport_maps = regexp:/etc/postfix/transport.regexp

transport.regexp:
/^FOO@example\.jp$/i smtp:xxx.xxx.xxx.xxx
/^foo@example\.jp$/i error:

maillog:
Aug 17 21:38:37 xxx postfix/smtpd[15274]: connect from c.example.jp[192.168.0.1]
Aug 17 21:38:38 xxx postfix/smtpd[15274]: NOQUEUE: reject: RCPT from c.example.j
p[192.168.0.1]: 550 <FOO@example.jp>: Recipient address rejected: Address is undel
iverable; from=<nobody@example.com> to=<FOO@example.jp> proto=ESMTP helo=<ZZZ>
Aug 17 21:38:38 xxx postfix/smtpd[15274]: disconnect from c.example.jp[192.168.0.1]
Aug 17 21:38:58 xxx postfix/smtpd[15274]: connect from c.example.jp[192.168.0.1]
Aug 17 21:38:58 xxx postfix/smtpd[15274]: NOQUEUE: reject: RCPT from c.example.j
p[192.168.0.1]: 550 <foo@example.jp>: Recipient address rejected: Address is undel
iverable; from=<nobody@example.com> to=<foo@example.jp> proto=ESMTP helo=<ZZZ>
Aug 17 21:38:58 xxx postfix/smtpd[15274]: disconnect from c.example.jp[192.168.0.1]