We are looking into moving into Exchange Online (but with an Exchange Hybrid server on premises).
What we currently couldn't find out is the backup topic. As there is no way to restore old deleted elements in Exchange Online we would like to use legal hold and prevent that the user can delete anything.
But how big could such a mailbox grow then? Isn´t there a quota restriction which blocks at some point the incoming emails? I think nobody from Microsoft would like to have a 500GB single mailbox on there server because the user isn´t deleting anything in the past 10 years.