Tracking the bewildering array of clients and different Internet browsers that are being used to send email on Exchange can be an interesting challenge especially when your clients are in the cloud. One Mapi property that gets set on the SentItem in the senders Mailbox can help answer some of these questions where logs may not be available. The Named Mapi property clientinfo eg
So if you write a script that accesses this property on messages in the SentItems folder you can build a report of the different sending methods that are being used and also different properties such as the browser version from the agent string being stored eg something like this
I've put a script on GitHub here https://github.com/gscales/Powershell-Scripts/blob/master/GetMailUserAgentsv2.ps1 that can mine the data in the property correctly taking into account the ; used as a separator while also dealing with data enclosed in ( ) (it does this by just doing a character by character parse rather the splitting or using Regex). The script will separate the Primary ClientType and extra client information (eg browser agent string) from the clientinfo property.
The script has a few different options by default it will evaluate every message in the SentItems folder if you use the -days switch you can specify how many days to look back eg
Get-MailUserAgents -Mailbox user@domain -days 14
Will look at messages for the last 14 days, if you want to evaluate messages in the Inbox you can use the -Inbox switch which will change the target folder to the Inbox.