[Exim] temporarily rejected RCPT

Dear Sirs.

2004-02-17 18:33:56 H=(mywankers.com) [66.9.135.151] F=<aburrell_ji@???> temporarily rejected RCPT <dm
ac@???>: Could not complete sender verify

I'm regulary getting the SPAM of this kind.

How can Exim be configured to generate 550 (permanent) instead of 451 (temporarily)?

Thanks!