Some mail server don't follow the RFC and so they have problems with Greylisting.

The most common problems are:

• An IP address pool where every connect uses a different IP address
• A unique sender for every connect
• The second connect is within seconds after the first one,
  but the third connect is late, sometimes too late
• Does not reschedule at all
• Reschedules in a very long interval that cause a long delay
• A strange scheduling ( there is no pattern in the schedule )

By default XWall excludes known misbehaving mail server from Greylisting and
the list below shows you which host names and IP addresses are excluded.

If you don't want the default exclusion then you can disable it by adding the line

InboundGreyListDefExclude=False

to XWall.ini

Note: You may add your own exclusions in Options->Spam->Greylisting->Exclude

Exclusion based on Host names and IP Addresses