I'm seeing a variation of this which began after upgrading from 14.5 to 15.0.5962. The SmarterMail directory had always been excluded from virus scanning, however as a test I disabled antivirus completely and the errors continue. However, the emails are spam checked and properly delivered following this.
[2016.05.09] 10:07:58  Error adding message to spool stat tracker: The process cannot access the file 'd:\SmarterMail\Spool\SubSpool5\53110042.eml' because it is being used by another process.
[2016.05.09] at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
[2016.05.09] at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy, Boolean useLongPath, Boolean checkHost)
[2016.05.09] at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, FileOptions options, String msgPath, Boolean bFromProxy)