Suppose the joined file can not be opened. Trying to run or open the joined file only results in an error message. What can be the cause? Very likely one of the split parts has been corrupted during download.
To find out whether file-corruption is the actual cause of the error message, generate a checksum of both files (the original file, and the joined file) and compare these checksums.
If the checksums are not the same, the joined file is corrupted.
Some of the possible causes of corruption of the joined file are:
(1) one or more split parts where damaged download or during the email sending process, or
(2) your friend's harddisk has one or more bad areas which caused one or more of the split parts and/or the joined file to become damaged, or
(3) the file has been infected by a virus on your friend's computer after joining, or
(4) your friend did not join all split parts.
(5) you tried to join files which are not part of a collection of split files created by HJSplit.