Were you able to open very large emails before you installed CSSU? (your post suggests you are not sure) does this apply to all emails or just a 'few'?

My bet is, the mails are Very large and actually reach the maximum number of characters (wich is a lot anyhow) although I don't know exactly what that value is. It happens to me sometimes when my friends send me (and many other friends, 20-30ish) and everyone responds with reply to all, often starting conversations via mail :P.

Don't worry if thats the case, the Iphone users amongs them start whining about maximum characters long before ny N900. In a case such as mine, you could delete old replies and keep on conversating/spamming but it hopefully only hides the oldest characters of the email, meaning its either a LONG read or old replys anyways