Migrating from Exchange 2010 where I have a server side setting for all mailboxes to save sent items to both shared mailbox and sender's mailbox:
On Exchange Online, it seems this option is only per mailbox - is there a way to set the tenant wide default to 'on' rather than 'off'?
Nope, it's per-mailbox, and covers User and Shared mailboxes, and no way to set a tenant-wide default value. It's easy to automate via PowerShell though, and a prime candidate for one of the actions in a "enable mailbox" script.
Thanks Vasil, that's what I suspected but couldn't find any official doco either way (not even around manually doing it via docs.microsoft.com, just the blog post from 3 years ago!).