要解决ASP.NET Core Identity的Yahoo电子邮件声明未收到的问题，您可以尝试以下代码示例：

1. 在Startup.cs文件的ConfigureServices方法中，添加以下代码以配置电子邮件服务：

using Microsoft.AspNetCore.Identity.UI.Services;
using Microsoft.Extensions.DependencyInjection;
using System.Net;
using System.Net.Mail;

public void ConfigureServices(IServiceCollection services)
{
    // 其他配置代码...

    // 添加电子邮件服务
    services.AddTransient();

    // 其他配置代码...
}

2. 创建一个名为EmailSender的新类，并实现IEmailSender接口。在该类中，添加以下代码来发送电子邮件：

using Microsoft.AspNetCore.Identity.UI.Services;
using System.Net;
using System.Net.Mail;
using System.Threading.Tasks;

public class EmailSender : IEmailSender
{
    public async Task SendEmailAsync(string email, string subject, string htmlMessage)
    {
        var fromAddress = new MailAddress("your-email@example.com", "Your Name");
        var toAddress = new MailAddress(email);
        const string fromPassword = "your-email-password";
        string smptServer = "smtp.mail.yahoo.com";
        int port = 587;

        var smtp = new SmtpClient
        {
            Host = smptServer,
            Port = port,
            EnableSsl = true,
            DeliveryMethod = SmtpDeliveryMethod.Network,
            UseDefaultCredentials = false,
            Credentials = new NetworkCredential(fromAddress.Address, fromPassword)
        };

        using (var message = new MailMessage(fromAddress, toAddress)
        {
            Subject = subject,
            Body = htmlMessage,
            IsBodyHtml = true
        })
        {
            await smtp.SendMailAsync(message);
        }
    }
}

请确保将your-email@example.com和your-email-password替换为您自己的电子邮件地址和密码。

这样，当ASP.NET Core Identity发送电子邮件时，它将使用您配置的Yahoo电子邮件服务器来发送声明电子邮件。