“In aviation, a chain of events, often called the error chain, is a term referring to the concept that many contributing factors typically lead to an accident, rather than one single event†(from Wikipedia)
I like that little chain of events quote that CEO Alex Quilici used in his post today about the issue and resolution. Apparently there were approximately 15,000 people who had made the update from the old version (1.8.3) to the new version (2.0.45). The later of the two is the one that was currently in the market. After updating there was an issue that YouMail was unaware of. The app was polling the network for voicemail continuously and putting strain on the network along with causing significant battery drop.
The second thing that got this app pulled came down to lack of communication. T-Mobile did try to contact YouMail to address the issue. However, a T-Mobile engineer sent an email to the generic customer support team who responded the issue was resolved in the most recent update. Which it had been, they just weren’t aware that the update from old to new had a glitch.
Finally, after not hearing back from YouMail, T-Mobile took the data to Google and showed them what was going on and that the developing company is not being responsive to the issue. That is what lead to Google shutting down the app from the market.
YouMail went through the appeal process still not understanding why it had been removed. After a cordial phone conversation with T-Mobile they worked it out. The app is now back in the market for you to enjoy. If you happen to be on the old version still, you are urged to update the app. I might recommend uninstalling the old one first and then installing the new one to help avoid conflicts.
There is a lot more information to read regarding what had happened. You might want to take a read of it over at the YouMail Blog. They even go on to offer up some tips for developers to help prevent unnecessary removal and long appeal processes.  The most important thing is that the application is back in the market.