(1) Sender creates SSL connection to middleman W

(2) Middleman W validates sender (using what credentials?)

(3) Sender creates and sends message M for some recipient R to W

(4) Sender sends authentication credentials for R to W

(5) Middleman W sends plaintext message to recipient R

 - contains SSL link to middleman W

 - contains a message password P, which the middleman "forgets" after sending

(6) recipient R creates SSL connection to middleman using link from (5)

 - provides P
 
 - which enables W to obtain the credentials/message intended for R

(7) middleman authenticates R using the credentials obtained in (4)

(8) if authentication in (7) succeeds, W displays message to R (over
    SSL link)