C# 服务器响应为：MFE代理上的请求操作中止，SMTP服务器不可用。”；发送电子邮件时

我正在使用下面的代码片段发送电子邮件，它在我的一个应用程序上运行良好，但在另一个应用程序上从未运行过

    try
    {
        var message = GetMessage(emailMessage);

        var emailCredentials = _appSettings.EmailConfiguration;

        SmtpClient client = new SmtpClient("smtp.gmail.com")
        {
            Port = 587,
            UseDefaultCredentials = false,
            DeliveryMethod = SmtpDeliveryMethod.Network,
            Credentials = new NetworkCredential(emailCredentials.SenderAddress, emailCredentials.SenderPassword),
            EnableSsl = true,
            Timeout = 300000
        };

        client.Send(message);

    }
    catch (Exception ex)
    {
        _logger.Error(ex);
    }

private MailMessage GetMessage(EmailMessage emailMessage)
{
    var message = new MailMessage();

    message.Subject = emailMessage.Subject;
    message.Body = emailMessage.Body;
    message.From = new MailAddress(_appSettings.EmailConfiguration.SenderAddress, _appSettings.EmailConfiguration.SenderName);
    message.Sender = new MailAddress(_appSettings.EmailConfiguration.SenderAddress, _appSettings.EmailConfiguration.SenderName);
    message.IsBodyHtml = true;

    emailMessage.Recipients.ForEach(recipient => message.To.Add(new MailAddress(recipient.EmailAddress, recipient.Name)));

    if (!emailMessage.Bcc.IsNullOrEmpty())
    {
        emailMessage.Bcc.ForEach(recipient => message.Bcc.Add(new MailAddress(recipient.EmailAddress, recipient.Name)));
    }

    if (!emailMessage.Cc.IsNullOrEmpty())
    {
        emailMessage.Cc.ForEach(recipient => message.CC.Add(new MailAddress(recipient.EmailAddress, recipient.Name)));
    }

    return message;
}

两个应用上的凭据相同，但在第一个应用上运行时，它抛出

服务器响应为：请求操作在MFE代理上中止，SMTP服务器不可用。

从那以后，我一直在研究是什么原因导致了这种情况，但我无法找出原因，我甚至将电子邮件配置从Gmail更改为Zoho，但仍然存在相同的错误。我在网上找不到任何有意义的东西，请帮忙