I have been using http://tempmail.de for a while. Its not blocked by any websites (From the ones that I have tried registering with. They are quite a few) And the best part is that you don't need to register an account to use the mailbox. You just need to visit the website and would be presented with a mailbox. You may get as many mailbox you need, by opening it in Private Window, every time.
@nikitsaraf: That's the precise reason we made it. It's really hard for normal users to share math/engineering formulas or derivations on the web. we though we will simplify it a bit.
