I cannot progress the Blackmail in Whitechapel quest

[omniforion]

So the other night I was about to continue the quest and I went to check the mailbox for the letter. I found it but before I read it I decided to go to sleep instead, so I quit out of the game. Today, I went to check for the mail as the box had not been ticked off and the letter was no longer there. I went to sleep in the game to see if it would appear but that didn't fix it.

[StarlightRac]

...ck you inventory reread everything there. just because you pick up items doesnt mean you read it; always make it habit to go into inventory after you find notes/letter/etc and read them. also do the same to items you find(alot of times there is notes that have been put into said items).