I don't run OS2008 yet, but my understanding is that the e-mail client is the same old one as in OS2007. If so, then the reason for the startup problem is that a sub-process of the e-mail client is still running. This blocks the startup. The reason for this is one of two:
a) The periodic e-mail updater is running. If so, a retry a few seconds later will succeed.
b) It was in the process of communicating with the server when the network was disconnected. For some reason this will leave the process hanging, and the e-mail client can't be started no matter what.

The fix for this is simple: In an xterm, enter 'killall enprocess' (without the quotes). After this everything should be ok. It's a common enough operation that I have added it to the load-applet-run applet's functions (by clicking 'Run', entering 'killall enprocess' and ticked the 'save' option.) Now I just hit load-applet-run and click my 'fixmail' entry. With this the problem is solved, at least it's no bother anymore (until I figured out the above I had to reboot the device.. not good.)