Question: If we wanted to have a "maintainable" archive of the emails sent by our app in a single file, what would be the best technique?
Answer: Perhaps save them in an XML file with this format:
…
…
The intent is to minimize the parsing so as the file grows you’re not parsing each individual email simply to load the file. You might save key information (date,subject,to) in XML nodes, but each email’s source is only parsed on an as-needed basis.
An example in C# is located here: https://www.example-code.com/csharp/saveSentEmailToXml.asp