Yes PST and OST Outlook files are not good when they get very large can become corrupted. You generally have to end up splitting a Large OST into multiple PST files which is a pain.
That’s one reason I imported all my old eg: Outlook 12GB files into eM Client (Via the Import whole account) option years ago where so far i haven’t had issues with synching email etc. So i would try importing your 50GB OST file.
If you have any issues importing the one 50GB OST file, then try splitting the OST into eg: two 25GB PST files and do the Import again.
See below thread post from Olivia Rust (eM Client support) on importing an OST file.
Note:- Make a copy of the original OST file first just incase you have any issues when importing.
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Olivia_Rust eM Client Support
Hello,
importing OST files directly is not supported and we do not plan to have this option in the future.
OST files are primarily meant for offline storage of data to later synchronize with the server. As such, data in a single file could be incomplete.
However, if you choose the option to Import data from Microsoft Outlook>Import whole accounts , the data will be loaded from both PST and OST files available.
Regards,
Olivia