In this article I will explain with an example, how to send email with multiple attachments using MailKit library in ASP.Net WebForms using C# and VB.Net.
Installing MailKit package
You will need to install the MailKit package, for details on installation please refer Install MailKit from Nuget in Visual Studio.
Mail Server Settings in Web.Config file
The following Mail Server settings need to be saved in the Web.Config file.
    <smtp deliveryMethod="Network">
          port="587" />
HTML Markup
The HTML Markup consists of:
TextBox – For capturing the values of Recipient Email address, Subject, Body, Gmail account email address, Gmail account password.
FileUpload – For attaching multiple files as attachment to the email.
The FileUpload has been assigned with AllowMultiple attribute for multiple file selection.
Button – For sending email.
The Button has been assigned with an OnClick event handler.
<table border="0" cellpadding="0" cellspacing="0">
        <td style="width: 80px">To:</td>
        <td><asp:TextBox ID="txtTo" runat="server"></asp:TextBox></td>
        <td><asp:TextBox ID="txtSubject" runat="server"></asp:TextBox></td>
        <td valign="top">Body:</td>
        <td><asp:TextBox ID="txtBody" runat="server" TextMode="MultiLine" Height="150" Width="200"></asp:TextBox></td>
        <td>File Attachment:</td>
        <td><asp:FileUpload ID="fuAttachment" runat="server" AllowMultiple="true" /></td>
        <td>Gmail Email:</td>
        <td><asp:TextBox ID="txtEmail" runat="server"></asp:TextBox></td>
        <td>Gmail Password:</td>
        <td><asp:TextBox ID="txtPassword" runat="server" TextMode="Password"></asp:TextBox></td>
        <td><asp:Button Text="Send" OnClick="SendEmail" runat="server" /></td>
You will need to import the following namespaces.
using MimeKit;
using MailKit.Net.Smtp;
using System.IO;
using System.Configuration;
using System.Net.Configuration;
Imports MimeKit
Imports MailKit.Net.Smtp
Imports System.IO
Imports System.Configuration
Imports System.Net.Configuration
Sending email with multiple attachments using MailKit
MimeMessage class properties
Following are the required properties of the MimeMessage class.
From – Sender’s email address.
To – Recipient(s) Email Address.
CC – Carbon Copies. (If any)
BCC – Blind Carbon Copies. (If any)
Subject – Subject of the Email.
Body – Body of the Email.
Attachments – Attachments. (If any)
ReplyTo – ReplyTo Email address.
SmtpClient class methods
Following are the methods of the SmtpClient class.
Connect – The connection to the SMTP Server is established using the domain and the port number.
Authenticate – The username and password of the SMTP Server is authenticated.
Send – The MimeMessage object is passed to it and the email is sent.
Disconnect – Disconnects the connection with SMTP Server.
Sending Email using MailKit
When the SendEmail Button is clicked, the Recipient email address (to), the Sender email address (from), Subject, Body and Attachment details are fetched from their respective fields and are set into the respective properties of the object of the MimeMessage class.
Setting Body of Email
For Body and Attachments, an object of Builder class is created. The Body of the email is Text (Non HTML) hence it is set into the TextBody property of the Builder class object.
Attaching multiple Files
The FileUpload is checked for Attachments and if it has Files then a FOR EACH loop is executed over the posted Files and inside the loop each File is added as Attachment to the Builder object.
Sending Email
Then, an object of the SmtpClient class is created and the values of Host and Port are fetched from the SMTP section of the Web.Config file and are passed as parameter to the Connect method.
And, the methods of the Mail Server such as ConnectAuthenticateSend and Disconnect are executed and a success message is displayed in JavaScript Alert Message Box using RegisterStartupScript method.
protected void SendEmail(object sender, EventArgs e)
    string to = txtTo.Text;
    string from = txtEmail.Text;
    string password = txtPassword.Text;
    string subject = txtSubject.Text;
    string body = txtBody.Text;
    IList<HttpPostedFile> postedFiles = fuAttachment.PostedFiles;
    //Read SMTP section from Web.Config.
    SmtpSection smtpSection = (SmtpSection)ConfigurationManager.GetSection("");
    string host = smtpSection.Network.Host;
    int port = smtpSection.Network.Port;
    using (MimeMessage mm = new MimeMessage())
        mm.From.Add(new MailboxAddress("Sender", from));
        mm.To.Add(new MailboxAddress("Recepient", to));
        mm.Subject = subject;
        BodyBuilder builder = new BodyBuilder();
        builder.TextBody = body;
        if (fuAttachment.HasFile)
            foreach (HttpPostedFile postedFile in postedFiles)
                string fileName = Path.GetFileName(postedFile.FileName);
                builder.Attachments.Add(fileName, postedFile.InputStream);
        mm.Body = builder.ToMessageBody();
        using (SmtpClient smtp = new SmtpClient())
            smtp.Connect(host, port);
            smtp.Authenticate(from, password);
    ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('Email sent.');", true);
Protected Sub SendEmail(ByVal sender As Object, ByVal e As EventArgs)
    Dim [to] As String = txtTo.Text
    Dim from As String = txtEmail.Text
    Dim password As String = txtPassword.Text
    Dim subject As String = txtSubject.Text
    Dim body As String = txtBody.Text
    Dim postedFiles As IList(Of HttpPostedFile) = fuAttachment.PostedFiles
    'Read SMTP section from Web.Config.
    Dim smtpSection As SmtpSection = CType(ConfigurationManager.GetSection(""), SmtpSection)
    Dim host As String = smtpSection.Network.Host
    Dim port As Integer = smtpSection.Network.Port
    Using mm As MimeMessage = New MimeMessage()
        mm.From.Add(New MailboxAddress("Sender", from))
        mm.[To].Add(New MailboxAddress("Recepient", [to]))
        mm.Subject = subject
        Dim builder As BodyBuilder = New BodyBuilder()
        builder.TextBody = body
        If fuAttachment.HasFile Then
            For Each postedFile As HttpPostedFile In postedFiles
                Dim fileName As String = Path.GetFileName(postedFile.FileName)
                builder.Attachments.Add(fileName, postedFile.InputStream)
        End If
        mm.Body = builder.ToMessageBody()
        Using smtp As SmtpClient = New SmtpClient()
            smtp.Connect(host, port)
            smtp.Authenticate(from, password)
        End Using
    End Using
    ClientScript.RegisterStartupScript(Me.GetType(), "alert", "alert('Email sent.');", True)
End Sub
Possible Errors
The possible errors (exceptions) occurring while sending email with MailKit in .Net are covered in the following article.
Email Form
Send Email with multiple attachments using MailKit in ASP.Net WebForms
The received email
Send Email with multiple attachments using MailKit in ASP.Net WebForms