All of the emails in the email directories contain one of the following at the end of the filename

$ ls cur/ | cut -d: -f 2 | sort | uniq -c
54 2,               <- Not Read
12 2,ab             <- Not Read
83 2,S              <- Read
61 2,Sab            <- Read

The first two “2, and 2,ab” mean that the message has not been read. The bottom 2 “2,S and s,Sab” mean that the message has been read or “seen?”. Guess that is what the S is for. Not sure what ab is for.