Too often for too long...
There have been blacklisting problems with TSO ever since I was sold to them a few years ago - but the last six months, things have become chronic with Microsoft. If you keep an eye on TSO's Status page (https://help.tsohost.com/status) you see a pretty continuous stream of blacklist incidents, which seem to be triggered by their mail servers failing to implement the security requirements which Microsoft set as the minimum for outlook.com and Hotmail to accept incoming connections. I have no idea whether there's any actual spamming going on, but I don't think the blacklisting results from customer complaints to MS - I think it's to do with sender policy and security implementation at the server level.
I'm not sure of the technical specifics, but I know that my own TSO server was blacklisted for over a week before it was even listed as such on their status page, and it took another week for TSO to get it delisted. So that was a fortnight during which some of my customers couldn't send messages to some of their customers - and that was by no means the first time, with incidents like this dating back to July.
So I've decided to move on, and am currently biting the extremely sour bullet of moving everything to another provider. It's a de-crufting opportunity, and I should probably do it more often - but as a purveyor of budget hosting services to SMEs, it's a bunch of unpaid work I could really do without.