|Building The Network You Need With PF, The OpenBSD Packet Filter: BSDCan 2016, Ottawa, Canada, June 8th 2016
Greylisting inevitably means delay for the initial message from a new correspondent.
Some sites are misconfigured; do not retry or retry way too fast (seconds)
Others have many outgoing MXes and random selection for retry
Some people are simply too impatient
Solution: use nospamd