I have noticed that some of my users (less that 5%) have been experiencing a "GroupWise Error 8503. Could not connect to the specified network address. Either the server is down, the network is slow or an invalid address and port combination was specified". The affected users are on different PO's and geographic locations, so I don't anything in common with that, since most others are not affected. It is also strange that many of the affected users only seem to see the "8503" problem when switching between their calendar and mailbox. I upgraded users desktop clients that were on GW8 to GW2012sp1 and that has not resolved the issue (our backend servers are Linux Sles 11sp2 with GW2012sp1). Running user/mailbox maintenance also has no affect. I've found references to this type of error, but only in reference to proxy access issues, so not the same context as I'm seeing it. Any suggestions would be appreciated.