Hi @Aran Billen ,
Thank you for posting to Microsoft Community.
As you mentioned, you could get the email activity report which includes the total sent count of users via Microsoft 365 Admin Center.
However, I am afraid there's no such a feature to select internal and external emails report separately at present.
You could run the following command to get the number of external/internal emails sent in the last 30 days by specific users:
Get-MessageTrace -RecipientAddress <user_email_address> -StartDate (Get-Date).AddDays(-30) -EndDate (Get-Date) | Where-Object {$_.SourceContext -notlike “*onmicrosoft.com 8”} | Select-Object RecipientAddress, SenderAddress | Group-Object RecipientAddress | Select-Object Name, Count
Then please kindly replace the “notlike” in the command to “like” to check the internal emails.
You can repeat the command for each user and combine the results to get a report of the number of external/internal emails sent by all the users you are interested in.
Also, I've just tried submitting a suggestion via the dedicated feedback portal for Exchange Online. I'll leave the link below so that you or other community members can also add vote or comment there. Let's hope this kind of report can be added in the future. https://feedbackportal.microsoft.com/feedback/idea/f6386999-5e86-ed11-a81b-6045bdaf6a9e
If my answer is any helpful to you, please mark it as the answer so that other users can refer to it. Thank you for your support and understanding.