File Extensions of Files in the Spool

The file extension also indicates the type of file.

.smd and .smp file extensions indicate regular mail messages being processed by SMTP.
.fwd and .fwp file extensions indicate forwarded messages.
.lst file extensions indicate messages to subscribers of a list server mailing list.
.tmp are Web Messaging.
.gse and .gsp file extensions indicate error messages being returned to the senders. These are usually generated by the server (postmaster)

Files that contain a tilde (~) in the file extension, such as .~mp and .~md, are locked files that are in process. These files also have an underscore as the first character in the file name.