要安装Mailkit并设置IEmailSender，可以按照以下步骤进行操作：

步骤1：安装Mailkit NuGet包 在Visual Studio中打开项目，右键单击项目名称，选择“管理NuGet程序包”。 在NuGet程序包管理器中搜索“Mailkit”，然后点击“安装”按钮以安装Mailkit。

步骤2：设置IEmailSender接口 在项目中创建一个类，并实现IEmailSender接口。以下是一个示例代码：

using MailKit.Net.Smtp;
using MailKit.Security;
using Microsoft.Extensions.Options;
using MimeKit;
using System.Threading.Tasks;

public class EmailSender : IEmailSender
{
    private readonly EmailSenderOptions _emailSenderOptions;

    public EmailSender(IOptions emailSenderOptions)
    {
        _emailSenderOptions = emailSenderOptions.Value;
    }

    public async Task SendEmailAsync(string email, string subject, string message)
    {
        var emailMessage = new MimeMessage();
        emailMessage.From.Add(new MailboxAddress(_emailSenderOptions.SenderName, _emailSenderOptions.SenderEmail));
        emailMessage.To.Add(new MailboxAddress("", email));
        emailMessage.Subject = subject;
        emailMessage.Body = new TextPart("plain") { Text = message };

        using (var client = new SmtpClient())
        {
            await client.ConnectAsync(_emailSenderOptions.SmtpServer, _emailSenderOptions.SmtpPort, SecureSocketOptions.StartTls);
            await client.AuthenticateAsync(_emailSenderOptions.SmtpUsername, _emailSenderOptions.SmtpPassword);
            await client.SendAsync(emailMessage);
            await client.DisconnectAsync(true);
        }
    }
}

public class EmailSenderOptions
{
    public string SenderName { get; set; }
    public string SenderEmail { get; set; }
    public string SmtpServer { get; set; }
    public int SmtpPort { get; set; }
    public string SmtpUsername { get; set; }
    public string SmtpPassword { get; set; }
}

步骤3：配置EmailSenderOptions 在appsettings.json文件中添加以下配置：

"EmailSenderOptions": {
    "SenderName": "Your Sender Name",
    "SenderEmail": "Your Sender Email",
    "SmtpServer": "Your Smtp Server",
    "SmtpPort": 587, // or your desired port
    "SmtpUsername": "Your Smtp Username",
    "SmtpPassword": "Your Smtp Password"
}

步骤4：注册IEmailSender服务 在Startup.cs文件的ConfigureServices方法中添加以下代码：

services.Configure(Configuration.GetSection("EmailSenderOptions"));
services.AddTransient();

现在，您已经成功安装Mailkit并设置了IEmailSender接口。您可以在需要发送电子邮件的地方注入IEmailSender，并使用SendEmailAsync方法发送电子邮件。