We've implemented a new change in response to some very persistent and highly annoying spam going on in the private messages. The spam was clearly handled by a bot, creating around 600 accounts in the last 24 hours to send spammy private messages out promoting their junky little travel forum (seriously it is total trash, I had a look).
So, in response to this, from now on, in order to get access to your message center you will need to verify your email address. This is a simple process. You get an email sent to you. You click on a verification link and voilá, everything works again and we know with quite a bit more certainty that you are human. I'm sure it can be gamed, but it's just that little bit harder.
We are not making this requirement for those users who are Full Member or over, since it would be very hard to get to that point anyway as a bot.
It's a shame we've had to take this step, since we do like to avoid such things, but this kind of spam just has to be dealt with and this is the most effective solution we could come up with.