在Asp.Net Core Identity中使用SendGrid发送邮件时，如果遇到邮件未发送的问题，可以按照以下步骤解决：

1. 确保已经正确配置SendGrid的API密钥。可以在SendGrid网站上创建一个账户并获取API密钥，然后在程序中将密钥配置到appsettings.json文件中：

"EmailSettings": {
  "SendGridApiKey": "YOUR_SENDGRID_API_KEY"
}

2. 在Startup.cs文件中的ConfigureServices方法中添加SendGrid的服务依赖：

services.AddTransient();

3. 创建一个实现IEmailSender接口的SendGridEmailSender类，并在该类中实现发送邮件的逻辑。可以使用SendGrid的官方包SendGrid.AspNetCore来简化操作：

public class SendGridEmailSender : IEmailSender
{
    private readonly EmailSettings _emailSettings;

    public SendGridEmailSender(IOptions emailSettings)
    {
        _emailSettings = emailSettings.Value;
    }

    public Task SendEmailAsync(string email, string subject, string message)
    {
        return Execute(_emailSettings.SendGridApiKey, subject, message, email);
    }

    private Task Execute(string apiKey, string subject, string message, string email)
    {
        var client = new SendGridClient(apiKey);
        var msg = new SendGridMessage
        {
            From = new EmailAddress(_emailSettings.FromEmail, _emailSettings.FromName),
            Subject = subject,
            PlainTextContent = message,
            HtmlContent = message
        };
        msg.AddTo(new EmailAddress(email));

        return client.SendEmailAsync(msg);
    }
}

4. 确保在appsettings.json文件中正确配置发件人的邮箱地址和名称：

"EmailSettings": {
  "FromEmail": "YOUR_EMAIL_ADDRESS",
  "FromName": "YOUR_NAME"
}

5. 在需要发送邮件的地方调用IEmailSender的SendEmailAsync方法发送邮件：

await _emailSender.SendEmailAsync(email, subject, message);

以上就是使用Asp.Net Core Identity和SendGrid发送邮件的解决方法。如果仍然遇到问题，可以检查SendGrid的API密钥和发件人邮箱配置是否正确，并确保网络连接正常。