This is PmailUnDup by Lamster Consulting. The original source was http://www.lamster.org/Software/default.html which no longer exists.
PmailUnDup was designed to remove duplicate emails however it also checks folder integrity and fixes indexes. It should work with any 4.x version of Pegasus Mail. It is supplied AS IS, without any warrantee as to merchantability or fitness for use.
The PmailUnDup230.zip file is the originally downloaded file.
The PMailUnDup_hlp_as_chm.zip file contains the original help file compiled as a .chm.