Hi jmendo67,
I am afraid it is not feasible to change the message preview option to 3 lines for all computers using GPO or registry. Currently we may have to modify the settings manually or you can share the instructions of changing the settings to your users.
Besides, it is encouraged to submit your feedback via
Outlook UserVoice forum. It is a place for customers provide feedback about Outlook for Windows. What’s more, if a feedback is high voted there by other customers, it will be promising that Microsoft
Product Team will take it into consideration when designing the next version in the future.
Regards,
Yuki Sun
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