It's come to my attention that the ingame mail may be working incorrectly:

If you unfriend someone, they are still able to send mail to you yet you cannot reply back. I'm not sure if blacklisting removes this feature... but it still seems kinda strange that people can do this.

I'm not sure if it's working as intended, but if it is, then this opens up an avenue in which unfriended and even blacklisted players can still harass someone.