Hi @T Crha
If the requirement is to get external emails received within 30 days, you may need to use Historical Search (Enhanced summary report or Extended report in Exchange Admin Center).
If the external sender's domain can be confirmed, you can specify it directly in the search.
If the sender domain is not confirmed, to me you may have to search for all receive emails then filter the results with applications like Excel to exclude internal senders.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.