This is related to 450 - greylisted. It would be useful to have use the admin page (that doesn't do much right now) to include configuration options to allow admins to add/edit the error codes that will be ignored. I ran across a particular email address that had a system that was using a blacklisting service. Getting them to change was a pain and I was going to have an number of people signing up from that domain. I had to hard code in a check for a 554 error.
If this feature is added it would be handy to have it with respect to each of the checks (i.e. to:, from:). It is not too hard but I have not had a chance to code it up and check it yet.