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There are some servers using 450 error code for greylisting:
Resolving host name "xxxx"...
Connecting to host address "xxxx"...
S 220 xxxx ESMTP Postfix
C HELO xxxx
S 250 xxxx
C MAIL FROM:
S 250 Ok
C RCPT TO:
S 450 : Recipient address rejected: Greylisted, see http://isg.ee.ethz.ch/tools/postgrey/help/xxxx.html
This host states that the address is undetermined.
I think this one is using postfix with postgrey.. It would be better to match "Greylist", not the code itself.