I have spamfilter to retry when it receives a 4xx error code from a remote server and bounce if it gets a 5xx error code.

Someone reported an issue where there server was not working right for a couple of days, but there were no messages in the queue.

A check of the logs shows this:

So what does this mean?  The remote server returned an error 451 (temporary problem, so we should queue) and then bounced the message to the sender with a 550 error?

This case is a bit peculiar, as SpamFilter encountered a "user not found" when forwarding the email to the destination SMTP server. This initial forwarding is the one where SpamFilter behaves differently depending on the 4xx or 4xx errors. However SpamFilter received a "user not found" message, in which case SpamFilter needs to send an NDR (non-deliverable receipt) back to the sender. This NDR is forwarded to the destination SMTP server for delivery, but it is at this point that the " 451 Temporary local problem - please try later" is encountered. Errors received during NDRs are handles "less gracefully" by SpamFilter as continuing re-sending them could potentially cause email loops, so after one try they are usually dropped.

Please note that I may be mis-reading the log entry above, and would need to see the complete transaction (along with the log entries relative to the original email) to be more certain.