Modify Outlook Categories using IMAP

Question

How to Set Categories on Microsoft mail.

Answer 1

As far as I know categories are not available through IMAP in Exchange/Office365.

You can only set Flagged flag easily:

client.FlagMessageByUID(uid, Flag.Flagged);

Keywords are however added as a Keyword header to the message it self (Keywords: Green category,Orange category)

So they are easy to obtain (using IMail.Headers["Keywords"]), however are harder to update/modify.

Following code downloads an email modifies it by adding Keyword e-mail header and uploads it, deleting the original message:

using (Imap client = new Imap())
{
    client.ConnectSSL(server);
    client.UseBestLogin(user, token);

    client.SelectInbox();

    long uid = client.GetAll()[0];

    MessageInfo info = client.GetMessageInfoByUID(uid);

    byte[] eml = client.GetMessageByUID(uid);   
    IMail email = new MailBuilder().CreateFromEml(eml);

    email.Headers["Keywords"] 
        = "Green category,Orange category";

    client.UploadMessage(email, new UploadMessageInfo{
        Flags = info.Flags,
        InternalDate = info.Envelope.InternalDateString});

    // Delete original message
    client.DeleteMessageByUID(uid);

    client.Close();
}