Unable to send an email to multiple addresses/recipients using C#

I am using the below code, and it only sends one email - I have to send the email to multiple addresses.

For getting more than one email I use:

string connectionString = ConfigurationManager.ConnectionStrings["email_data"].ConnectionString;
OleDbConnection con100 = new OleDbConnection(connectionString);
OleDbCommand cmd100 = new OleDbCommand("select top 3 emails  from bulk_tbl", con100);
OleDbDataAdapter da100 = new OleDbDataAdapter(cmd100);
DataSet ds100 = new DataSet();
da100.Fill(ds100);

    for (int i = 0; i < ds100.Tables[0].Rows.Count; i++)
    //try
    {
        string all_emails = ds100.Tables[0].Rows[i][0].ToString();
        {
            string allmail = all_emails + ";";
            Session.Add("ad_emails",allmail);
            Response.Write(Session["ad_emails"]);
            send_mail();
        }
    }

and for sending the email I use:

string sendto = Session["ad_emails"].ToString();

MailMessage message = new MailMessage("[email protected]", sendto, "subject", "body");
SmtpClient emailClient = new SmtpClient("mail.smtp.com");
System.Net.NetworkCredential SMTPUserInfo = new System.Net.NetworkCredential("abc", "abc");
emailClient.UseDefaultCredentials = true;
emailClient.Credentials = SMTPUserInfo;
emailClient.Send(message);

The problem is that you are supplying a list of addresses separated by semi-colons to the MailMessage constructor when it only takes a string representing a single address:

A String that contains the address of the recipient of the e-mail message.

or possibly a list separated by commas (see below).

Source

To specify multiple addresses you need to use the To property which is a MailAddressCollection, though the examples on these pages don't show it very clearly:

message.To.Add("[email protected], [email protected]"));

The e-mail addresses to add to the MailAddressCollection. Multiple e-mail addresses must be separated with a comma character (",").

MSDN page

so creating the MailMessage with a comma separated list should work.

This is what worked for me. (recipients is an Array of Strings)

//Fuse all Receivers
var allRecipients = String.Join(",", recipients);

//Create new mail
var mail = new MailMessage(sender, allRecipients, subject, body);

//Create new SmtpClient
var smtpClient = new SmtpClient(hostname, port);

//Try Sending The mail
try
{
    smtpClient.Send(mail);
}
catch (Exception ex)
{
    Log.Error(String.Format("MailAppointment: Could Not Send Mail. Error = {0}",ex), this);
    return false;
}

This function validates a comma- or semicolon-separated list of email addresses:

public static bool IsValidEmailString(string emailAddresses)
{
    try
    {
        var addresses = emailAddresses.Split(',', ';')
            .Where(a => !string.IsNullOrWhiteSpace(a))
            .ToArray();

        var reformattedAddresses = string.Join(",", addresses);

        var dummyMessage = new System.Net.Mail.MailMessage();
        dummyMessage.To.Add(reformattedAddresses);
        return true;
    }
    catch
    {
        return false;
    }
}