My Blackberry used to be connected through BES to my work's email. Ever since I left, I got to keep the Blackberry and am now only using it for personal email. However, it still seems to want to connect through the BES. How do I turn this off? In particular, I often (but not always) get the error "A communication failure occurred with the selected Mobile Data Service." when using the browser.

I've already set the default browser configuration to "internet browser"
settings->options->advanced->browser
which seemed to have helped for some sites, but I'm still getting the error.

Anyone know how to disable MDS and BES completely from my Blackberry - I'm not in touch with the people who set it up in the first place and so am hoping I can fix it myself.

you should be able to remove the service books as required to stop this, but usually the handsets are locked as to a BIS personal or BES enterprise device, which isn't going to help you, you will also have your ex-companies IT policy on the device.

Setting browser to internet browser will incur costs, only if its running as 'blackberry browser' is the cost inclusive on your deal, which you would need to pay for personally, unless your ex-employer forgot to dump your number from their corporate package/account.

your carrier needs to allow you to have the data/internet package applied to your account, and the Blackberry Internet Service service book.