HTC have pushed out a second SMS hotfix for the HD2 today, described as a "new SMS function update". While the description is generic, it's believed that the new update might address ongoing SMS issues whereby outgoing messages fail to send and instead get stuck in the user's outbox.
"This update for HTC HD2 messaging lets you reach out to your friends and family with instant notifications. Keeping in touch has always been very important, and this update delivers just that: fast and reliable SMS messaging."
A previous hotfix released in mid-December was meant to sort the problem, but some users have reported continuing delivery failures even after updating their HD2. It's worth noting that you'll need a Windows PC in order to install the new software; if you've been having SMS issues and this update address them (or, conversely, fails to), let us know in the comments.