temporary mail refusal

[Harwood]

I am frequently getting a temporary mail refusal message from the server. What is causing this?

[Code Crafters]

This message is caused by the "Grey Listing" SPAM filter. This is very effective at eliminating SPAM mail. It works by refusing the first mail with a temporary error for any sender / recipient pair (possibly uses subject too) that it hasn't seen before. Any normal mail server will simply resend the mail after several minutes. The second attempt, providing it's in the acceptance period (e.g. 1 minute to 3 hours - set in the SPAM settings), will be accepted and that sender / recipient pair will always be allowed first time from then on.

This SPAM filter will block around 80% of all SPAM on it's own without block legitimate mails although this can cause a small delay in delivery but if you change the acceptance window lower value to anything after 1 minute with the upper limit at something like 180 minutes (3 hours) or 360 minutes (6 hours), you should see minimum delay with a very effective reduction in SPAM filtering.