We had it turned on for 150 mailboxes and approx. 200k / in-out emails daily (heavy outbound due to confirmation and statements). As Marc said accuracy with grey-listing is very high - we had about 2 junks per thousand. By design there will be delays with it enabled.

 

As sales people noticed delivery delays with their sales leads (almost always with new email addresses, new domaines and new smtp servers which simply equals to worst case), we had to turn it off and the delays were obviously better but the accuracy worst, about 10 junks per thousands.

 

The decision really depends on the purposes of your email usage.

 

Cheers.

 

- Gregory