The Official SBS Blog : Cannot Reply To Old Emails Or Modify Old Calendar Items After PST Mail Migration: http://blogs.technet.com/sbs/archive/2009/05/21/cannot-reply-to-old-emails-or-modify-old-calendar-items-after-pst-mail-migration.aspx
Read the complete post at http://msmvps.com/blogs/bradley/archive/2009/05/22/the-official-sbs-blog-cannot-reply-to-old-emails-or-modify-old-calendar-items-after-pst-mail-migration.aspx