Fetchmail receives the "550 Spam refused" and reports that courier refused delviery then sends an RSET. Courier responds to the RSET but - it appears - often not for 20-70 seconds. When the delay is >60 seconds, the upstream ISP pop3 connection has timed out (it seems to have a 60 second timeout).
The first time a message is rejected, the response to the next command is delayed by 8 seconds.
If subsequent messages continue to be rejected, the delay gets progressively increased. The delay gets reset only when a message is succesfully accepted.
Therefore, when someone starts flinging spam at Courier, non-stop, the sender will be quickly tarpitted.
Solution: don't start throwing spam at Courier.
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