SMTP 具有加密权限的邮件 Outlook

Question

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When I send a mail with the option "Encrypt only" in Outlook

I receive a mail like this:

Currently, I use SMTPClient with MailMessage to send mail:

MailMessage message = new MailMessage();
message.From = from;
MailAddress to = new MailAddress(destString);
message.To.Add(to);
message.Subject = subject;
smtpClient.Send(message);

How can I modify my code to send a mail and receive it as the mail above?

After some researching, I found a solution by using Interop but I can't use it in my project.

var app = new Microsoft.Office.Interop.Outlook.Application();
var item = (Microsoft.Office.Interop.Outlook.MailItem)app.CreateItem(Microsoft.Office.Interop.Outlook.OlItemType.olMailItem);
item.To = abc@gmail.com;
item.Subject = "test";
item.Permission = Microsoft.Office.Interop.Outlook.OlPermission.olDoNotForward;
item.PermissionService = Microsoft.Office.Interop.Outlook.OlPermissionService.olWindows;
item.Send();

Answer 1

As far as I understand Outlook puts the message in a secure server (Office 365 in my case) and sends the recipient a link to open that message.

It is stated in this url , this feature is only for some licenses.

Microsoft 365 Message Encryption is part of the Office 365 Enterprise E3 license. Additionally, the Encrypt-Only feature (the option under the Encrypt button) is only enabled for subscribers (Microsoft 365 Apps for enterprise users) that also use Exchange Online.

I cannot find an API or library documentation, but you should try to communicate with your MS representative about this, so they can provide more information.