I was about to build a couple of BES 4 servers running Domin 6.5.4 collecting mail from a number of mail servers. I have just been informed that the Domino version to be installed on the BES should be the same as the lowest version on any of the mail servers ... in this instance we have some mail servers running 5.0.6a!! Failure to do this will cause big WAN issues due to the refreshing of Unread marks. Can anyone verify if this is the case as I don't really want to install 5.0.6a code instead of 6.5.4? Cheers

this was definitely a problem with version 4. But I have no information’s if this problem is solved with SP3, sorry...

Domino 5.0.x and R5 clients : The requirement to refresh the read/unread flags is by design. Since the feature (activated by the pressing the F9 key) is locally installed on the R5 client.
Domino 6.5.x and R6 clients: The code for the feature is installed on the server side, hence there's no need to use the F9 key to "sync" the read/unread flags. Works well with the wireless features of the BES v4.0 server.
In a mixed environment: The R5 mail template on a R6 server would still encounter some issues with read/unread flags.
Reason to upgrade to R6, unless the NAB is customized or use of some dev apps
presents a problem.

as dev said IBM changed the way domino handels the read/unread flags.
Installing the BES on a 6.x server results, that the BES is using an Domino 6 API Call
to get the read/unread flag snyc on the Mailserver. In case you try to sync a user on a Domino 5.x server. The old server doesn`t understand the API call and just ignores it.
Due the fact that BES tries to sync and doesn`t get an respond he send the API call again and again resulting in an high network use.

Thanks for the replies even if the news is not good!!!!