e-mail and delete files with FileSystemWatcher

I have this program which create files in some specific folder. There is a listner to that folder and it will e-mail the file to prefine mail address and then it will delete the file. This actually works fine when I test it for the first time. But time to time it gives this exception
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   at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)

   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)

   at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share)

   at System.Net.Mail.AttachmentBase.SetContentFromFile(String fileName, String mediaType)

   at System.Net.Mail.AttachmentBase..ctor(String fileName)

 & ...