G'Day
If you are using Outlook Express, I do not see why not, I regularly compress messages.
Basically what happens is as follows. When you delete a message, it no longer appears in your messages, but it is still in the mailbox on your hard disk. OE marks the message for deletion in the disk file but does not actually delete the message until you compress the file. So your mail files keep growing. What compression does is remove all emails marked for deletion and compresses the files removing all the unused space in the files.
If the actual files are too large (because you have never compressed them to remove empty space), when you do compress them, they become corrupted.
What you can do is:
Close OE.
Create a new folder.
Search your hard drive for all files ".dbx" - these are you mail files, inbox.dbx, sent items.dbx etc.
Open the folder containing the ".dbx" files and copy them to the new folder you created (this will backup the folders) - make sure you copy "folders.dbx" - this is the master file, without it your other .dbx files are useless.
Then you can open OE and then repeat the following for each folder:
Select folder (eg Inbox)|File|Folder|Compress Folder.
This will compress the folders one at a time.
If there are any folders that fail to compress, close OE and then copy all the folders back to their original location. You may want to consider consider creating new folders (in OE) and moving some of your mail to them - and then try compressing the files again.
If the deleted.dbx file becomes corrupted (it can become huge after a while) you could try (with OE closed) deleting the deleted.dbx file and then creating a new file called deleted.dbx - this sometimes works
Geoff