Windows live mail error 0X800C013E after feature 2004

I think you are making the migration to Thunderbird harder than you need to.  Just follow the process described at: https://support.mozilla.org/en-US/kb/switching-thunderbird .  You don't need to export anything from WLM; Thunderbird does all the work and directly imports from the directory structure and .eml files located at "\AppData\Local\Microsoft\Windows Live Mail\Storage Folders".  All of my .eml files in WLM are 21 characters long including the .eml extension.  If the long file names you are referring to are the path names rather than the file name, then you can choose to import all the .eml files in a single folder rather than including subfolders.  The ImportExportTools NG add-on in Thunderbird lets you specify the local folder or folders you want your .eml files imported to.  The path length and hard structure / virtual structure are not constraints to migrating to Thunderbird if you use the import process described in the link above.

Thunderbird is the email client that I tried unsuccessfully to migrate to 2 years ago.

WLM has two structures, a hard file structure that you can navigate in using File Explorer and a virtual one that maps into the hard structure. The folder names that you see in the WLM user interface come from the virtual structure. You can change the folder names and their position in the virtual structure but the file names and positions in the hard structure remain fixed. It is just the cross reference between the two structures which gets updated. You can delete an email in the virtual structure but the data remains in the hard structure as junk.

Filenames in the hard structure must not exceed 255 characters. Names in the virtual structure may be quite long but are concatenated down to 15 characters upon export.

In the perfect world, File Explorer would allow me to reduce in the export file the size of file names in the hard structure which have grown in size close to the limit of 255 characters. 

I have no proof, but I sensed that when the WLM message store is exported from one version of Windows to a newer version of Windows, part of the export file contains the cross reference which cannot be disected or patched with an editor. Thus with every successive export, the file names in the hard structure grow as the characters « /storage folder/«  are repeatedly added, and eventually cause the whole filename string to approach 255 characters. I have migrated from XP to Vista to Win8 and then to Win10.

I reduced the file name length of many files before I imported them into Thunderbird, I did not find my emails building into the hard structure tree that I expected. It just seemed to me that Thunderbird knew something about the old WLM cross reference, as if the cross reference had been encrypted into the export file and Thunderbird was picking this up and applying some of the layout of the old WLM hard structure against my will.

I had not come across this WLM fix until after I had already imported all my stored WLM email to Mozilla Thunderbird.  Thunderbird's website offers an import/export add-on that will import your WLM storage folders and saved emails.  I imported about 30,000 emails with no issues and the Thunderbird add-on preserved the directory structure.  I only had a minor problem with a few folder names.  On import, Thunderbird uses the folder names in the directory "\AppData\Local\Microsoft\Windows Live Mail\Storage Folders\Imported Folder" and a few of those folder names are slightly different than the corresponding folder names that appear in WLM.  I was able to easily correct those folder names manually from within Thunderbird.  I have been using Thunderbird now for a little over a week without any major issues and recommend it.  Thunderbird has a few quirks and takes a little getting used to, but since Microsoft no longer supports WLM, I knew I eventually had to migrate to something else.

Normally big and small updates to Windows 10 don’t upset WLM, except this time with version 2004. We were lucky that someone in the world knew how to work around this with a registry patch. I hope that we will continue to be as lucky in the future.

There are other free email clients available but it is not easy to export the WLM message store with its limit of 15 characters for a virtual folder name.

And I have had difficulty importing super large message stores into another client seeing it lock up after 20000 of my 30000 email were accepted.

Très grand Merci  pour vos conseils ..