C# sending authenticated mail through SMTP

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C#

SMTP

authentication

i'm having a little issue with my sendmail script. My mail can't be delivered to one of my internal distribution lists. Other DL are receiving the mail. I'm getting the returned message:

RESOLVER.RST.AuthRequired; Authentication required

My c# code looks like this (method SendMail belongs to a class named Plan):

public void SendMail(int supplierValue, List<Location> locationList, string meldingsNr, string date, string fromT, string tillT)
    {
    var mailFrom = "mail@mail.com";
    SmtpClient client = new SmtpClient("server ip",25);
    client.UseDefaultCredentials = false;
    NetworkCredential basicAuthInfo = new NetworkCredential("username", "password");
    client.Credentials = basicAuthInfo;
    client.EnableSsl = false;

    // Create some empty vars

    var toList = "";
    var ccList = "";
    var mailSubject = "";
    var mailBody = "";
    List<string> locationListString = new List<string>();
    List<string> locationListString2 = new List<string>();

    // Populate toList with var llString

    ** code removed **

    // switch on supplier Value to fill toList, ccList and mailSubject

    ** code removed **

    // Create mail message

    MailMessage message = new MailMessage();
    MailAddress mailAddress = new MailAddress(mailFrom);
    message.To.Add(toList);
    message.From = mailAddress;
    message.CC.Add(ccList);

    // set subject and encoding

    message.Subject = mailSubject;
    message.SubjectEncoding = Encoding.UTF8;

    // set body and encoding

    message.Body = mailBody;
    message.BodyEncoding = Encoding.UTF8;
    message.IsBodyHtml = true;

    try
    {
        // Send mail 
        client.SendCompleted += (s, e) =>
        {
            client.Dispose();
            message.Dispose();
        };
        client.SendAsync(message, null);

    } catch (SmtpException ex)
    {
        MessageBox.Show(ex.InnerException.Message);

    } catch (System.Exception ex)
    {
       MessageBox.Show(ex.InnerException.Message);
    }
}

What I have tried:

i tried applying this solution:
authentication - How can I make SMTP authenticated in C# - Stack Overflow[^]

Make sure you set SmtpClient.Credentials after calling SmtpClient.UseDefaultCredentials = false. The order is important as setting SmtpClient.UseDefaultCredentials = false will reset SmtpClient.Credentials to null.

but this didn't work. Is there anything else wrong with my code ?