Salesbox can only save those emails that Microsoft notify Salesbox about. Some emails are not shared by Microsoft. Since early February 2020 Microsoft has a Beta API available that can notify Salesbox about emails that was previously missed to notify about from Microsoft. Salesbox is also connected to that API to get the missing emails.
This is however not a guarantee that 100% of all sent/received emails are shared from Microsoft. What we have seen is that if your sent emails end up in the receivers Spam/Junk inbox it might risk that the email is not shared. However, there is no known list of reasons so we just want to highlight that some emails might not be saved to Salesbox due to these reasons.