There is a bug in the mail85.ntf template that makes it impossible to add ACL groups to “access & delegation” in your mailbox preferences. When trying to add an ACL group, you will get an error message saying: “The directory server is down or the user (or group) you want to grant access to your mail file could not be found in the server directory”. Adding other group types is possible, but it should also be possible to add ACL groups (which I think is logical).
A temporary workaround is changing the group type of the ACL group to “multi purpose” or “mail”, adding it to access & delegation and then switching it back to “ACL”.
The bug is described in SPR #RREN843DXM and according to IBM, this should be fixed in 8.5.3, coming in Q2 of 2011.