When greylisting is enabled, SmarterMail outputs the greylist.sbin file every 15 seconds. We have what I would guess is a moderately busy server. Over the past 7 days, it has recorded 253 GB of data transfer on the Reports -> Protocol page. I realize that Bandwidth Activity is not equal to Disk Activity, but there isn't a disk activity report I can use for comparison. Aside from indexing and logging, most bandwidth activity probably equates to a similar amount of disk activity, so it's a reasonable comparison.
Our greylist.sbin file is 25MB in size (and still growing a few MB per week). Since SmarterMail writes this file every 15 seconds (4 times a minute), just enabling greylisting causes 100 MB a minute, 6 GB an hour, 144 GB a day, and 1,008 GB a week in disk activity. If bandwidth activity approximates disk activity, just having greylisting enabled generates 4 times as more disk activity than all mail handling on the server itself!
Tim and Company love to talk about how disk efficient their product is, and regularly post Release Notes that start with EFFICIENCY: pointing out how much they've improved disk, CPU, or memory efficiency. I have brought this gross inefficiency to support's attention and they tell me it isn't a problem. I'm annoyed because for no good reason, this excessive disk activity will wear out my brand new server's SSD disks 4 times sooner than if they improved the efficiency of greylisting, perhaps by writing the file once an hour or once a day.
SmarterTools: Please make the frequency with which the greylist.sbin file is written Administrator-configurable so that those of us who care about disk efficiency and disk life can set this to an efficient setting.