grellis: Desktop Manager will work with all versions/combinations of versions....and there's not much really changed in 4.0 from 3.6. And if you don't use the Cert Sync from 4.0, or look at the logo, you probably won't notice a difference
kalch: There is no real downside to using the web client over Redirector. There are some upsides to it though...If there is a power outage at work and your computer turns off in the middle of the night, you still get email from the web client. You can log into your web client from any Internet enabled computer...etc.
As for your initial error, if you get a chance, talk to your network admins and see if they have an "intelligent" or "adaptive" spam filter on the Exchange server or even on the Firewall. From what it sounds like, while you were in the US, all your emails were coming from the handheld, instead of both the handheld and the desktop, and that means more than normal traffic going to the blackberry.net domain and coming from it. I"m wondering if this "intelligent" spam filter has now blacklisted the blackberry.net domain and that's why things have stopped working for you.
But, like I said, no real downsides, but there are upsides to using the web client over the Desktop Redirector.