Ditto
This particular problem is starting to grind me down. I too have the same problem: Office 2003 on one section of users encountering the 0x8004210a error and one section using office XP with no error.
All users are Symantec Enterprise clients, all users are connected to Ipswitch IMail, all users are connected via a 100mb network, the imail server is contained in a firewall DMZ.
Microsoft only admit that this affects office 2000/2002 and not above, nothing is found on either Symantec for enterprise or Ipswitch for IMAil.
I don't want to step back from Office 2003 as this is used by the editors and requires the functionality.
Any guidance would be appreciated.
Chris