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.