I have added the requested functionality. The store message behavior is now driven by mail.IsDraft property (which corresponds to value of the "X-Unsent" header).
To store a message in read-only state, just set MailMessage or MimeMessage property IsDraft to false and then store that mail.
Because, mail.IsDraft is false by default ("X-Unsent" header is missing by default), default behavior of store message was changed as well.
To store a message in editable state, either set mail.IsDraft to true or set the Ews.Settings.EnableIsDraftDetection option to false (default value is true).
Setting this option to false, the mail.IsDraft property is ignored and the mail is stored in editable format (which was previous behavior).
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If you have valid support contract and would like to have non-trial version, please let us know at email@example.com.