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.