I'm using V 1906 of Outlook 365 and am unable to get it to keep any zoom setting other than 100%. I can change it, but every time I open another email it reverts to the default. I found a post that said any version higher than 1901 will allow retention of settings but my version does not. I've also tried altering the pane sizes but those keep reverting to default also. The only thing that works is collapsing the folder pane, which is a PITA.