The letter you sent is signed by 1BitmixUUEGGACPeKciTkxr6TaLUnYgekM, meanwhile our address is 1BitmixerEiyyp3eTLaCpgBbhYERs48qza. So it is not our Letter of Guarantee, seems user been scammed by one of phishing websites.
Oops! I must have messed up somewhere while I was verifying one of my own letters. 
OP sent me the letter of guarantee, which I have forwarded to Bitmixer after they had responded to this thread.