MailKit SaslMechanismOAuth2

var cred = new UsernamePasswordCredential(PopUser,PopPass,Authority,AppID,options); 
// Scopesは["https://outlook.office365.com/POP.AccessAsUser.All","https://outlook.office365.com/SMTP.Send"]
var req = new TokenRequestContext(scopes);
var token = await cred.GetTokenAsync(req);
var oauth2 = new SaslMechanismOAuth2(PopUser, token.Token);

using (Pop3Client cli = new Pop3Client()){
    await cli.ConnectAsync("outlook.office365.com", 995, SecureSocketOptions.SslOnConnect);
    // AccessTokenで認証
    await cli.AuthenticateAsync(oauth2);
    int cnt = await cli.GetMessageCountAsync();
	・・・
}
using (SmtpClient scli = new SmtpClient()) {
    MimeMessage msg = new MimeMessage();
	・・・
    // SASLの認証済みフラグを落とす
    oauth2.Reset();

    await scli.ConnectAsync("outlook.office365.com", 587, SecureSocketOptions.StartTls);
    await scli.AuthenticateAsync(oauth2);
    await scli.SendAsync(msg);
    await scli.DisconnectAsync(true);
}