Firstly, before you do anything else - reboot the phone.
See if the same error occurs.
If it does, see if there is any difference made from being on a wireless connection, or using cellular data - try switching between the two.
Try visiting m.hotmail.com on your phone (in internet explorer) - can you access? If not, you may want to sync with Zune and do a backup, then try removing and re-adding your account.
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Rallicat
I just got this issue resolved on my phone. First off, after going nuts due to lack of tech support (and spending an hour on the phone with Microsoft), I did fin an HTC tech support site.
The tech was very helpful and had me do a factory reset (Settings/About/Reset Your Phone). I went through the step to set-up and everything has been working beautifully.