1. 检查SendGrid API Key是否正确配置，确保API Key可以访问SendGrid服务。

2. 确认SendGrid API Key已在应用程序中配置。

3. 确认SendGrid客户端已被注入，并检查是否已成功完成注入。

4. 检查邮件发送地址是否正确。可以将地址更改为另一个有效邮件地址，并尝试重新发送邮件。

以下是一些可能的SendGrid配置示例：

appsettings.json：

"SendGrid": {
    "ApiKey": "xxx",
    "FromName": "xxx",
    "FromEmail": "xxx"
}

Startup.cs：

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers();

    // Add SendGrid
    services.AddTransient((x =>
        new SendGridClient(Configuration.GetValue("SendGrid:ApiKey"))));
    services.AddTransient();
}

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    app.UseRouting();

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapControllers();
    });

    // Add SendGrid
    var sendGridOptions = Configuration.GetSection("SendGrid").Get();
    if (sendGridOptions != null)
    {
        app.UseSendGridEmail(new SendGridEmailOptions
        {
            ApiKey = sendGridOptions.ApiKey,
            FromName = sendGridOptions.FromName,
            FromEmail = sendGridOptions.FromEmail
        });
    }
}

SendEmailService.cs:

public interface ISendEmailService
{
    Task SendEmailAsync(EmailMessage emailMessage);
}

public class SendEmailService : ISendEmailService
{
    private readonly ISendGridClient _sendGridClient;

    public SendEmailService(ISendGridClient sendGridClient)
    {
        _sendGridClient = sendGridClient;
    }

    public async Task SendEmailAsync(EmailMessage emailMessage)
    {
        var from = new EmailAddress(emailMessage.From);
        var subject = emailMessage.Subject;
        var to = new EmailAddress(emailMessage.To);
        var plainTextContent = emailMessage.PlainTextContent;
        var htmlContent = emailMessage.HtmlContent;

        var msg = MailHelper.CreateSingleEmail(from, to, subject, plainTextContent, htmlContent);
        var response = await _sendGridClient.SendEmailAsync(msg);
    }
}