Is the email that is send out to the External users the Mendix application, or just any application )like Outlook). If the answer is Mendix, you should record the outgoing messages from Mendix and the incoming messages using a Marketplace module like: https://marketplace.mendix.com/link/component/1042
If the source of the mail is not Mendix, you need an additional tool or to adjust a setting on your current mail server. https://expertinsights.com/insights/the-top-10-email-archiving-solutions-for-businesses/ . Look for an REST API to retrieve the emails that were sent out.
Warning: in many countries, it is forbidden by law to tap the emails and record them. There are some great examples where you can record meta data on the emails without recording the content. See https://academy.mendix.com/link/paths/118/Text-Analytics-and-Translation-App and the sentiment detector.
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