unable to open

James <[email protected]> Mon, 30 Jul 2018 11:32:20 -0400
Newsgroups gmane.mail.exim.spamassassin
Message-ID <[email protected]>
archive-iterator: unable to open
****/Maildir/.Sent/cur/1532353080.M839526P771.gw,S=459,W=472:2,S: No
such file or directory

1532353080.M839526P771.gw,S=459,W=472:2,RS exists.
What is the 'S' file?