AGNC Investment (AGNC 0.94%) has one of the highest dividend yields in the market, sitting at about 16%. But with a stock price that's steadily declined the past few years, investors are right to ask: Is the payout sustainable, and more importantly, is the stock a buy today?For those unfamiliar, AGNC is a mortgage real estate investment trust (mREIT) that owns agency mortgage-backed securities (MBS), primarily guaranteed by Fannie Mae and Freddie Mac. Because these securities are backed by government agenci ...