SmtpClient仅在内部发送电子邮件时起作用

Question

我创建了一个发送电子邮件的网络表单。当toAddr中的电子邮件地址是例如johndoe@company.com时它可以工作，但如果我尝试johndoe@yahoo.com或johndoe@gmail.com它不起作用。电子邮件仅在内部发送。

我得到的错误：＆＃34; System.Net.Mail.SmtpFailedRecipientException：邮箱不可用。服务器响应为：5.7.1无法中继＆＃34;

SmtpClient client = new SmtpClient("mail.company.com");
MailAddress toAddr = new MailAddress("johndoe@yahoo.com");
MailAddress fromAddr = new MailAddress("notify@company.com");
String messageText1 = "<h4>Thank you.</h4>";

string messageSubject = "Confirmation";
MailMessage message = new MailMessage(fromAddr, toAddr);

message.IsBodyHtml = true;

message.Subject = messageSubject;
message.Body = messageText1; 

client.Send(message);