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Server response: 5.7.1 Failed to transmit repeater / Mailer Daemon

I reviewed the answer to the following question and found it very useful. However, I have another question regarding the behavior of SMTP servers. Mailbox Unavailable and Relay Configuration

Scenario: I have two smtp servers that I do not have access to, for example, credentials. When sending email from 1 server, I get the following error:

Mailbox not available. Server response: 5.7.1 Failed to transmit

Type: System.Net.Mail.SmtpFailedRecipientException:

Source: system

Stack trace: on System.Net.Mail.SmtpTransport.SendMail (sender MailAddress, MailAddressCollection recipients, String deliveryNotify, Boolean allowUnicode, SmtpFailedRecipientException & exception) with System.Net.Mail.SmtpClient.Send (MailMessage message)

very clear, i.e. smtp server could not send the message. However, on another server, the SendEmail method did not throw any exceptions, but I received a Mailer Daemon email message stating that:

This is the mail system in the host test. relay .host.name.changed.

I apologize to inform you that your message could not be delivered to one or more recipients. It is attached below.

For more help, send an email to postmaster.

If you do, include this issue report. You can remove your own text from the attached returned message.

Mail system

test1.test2@hotmail.com: host mx3.hotmail.com [xx.xx.xx.xxx] said: 550 Requested actions are not accepted: the mailbox is unavailable (in response to the RCPT TO command)

Question:

  • Why different behavior? that is, in one case, I get the daemon mailer, while in the other exception in the SendEmail method?

Besides,

  1. Specifically, how do daemon mail messages work? Because according to the second message, it looks like it was a relay sent by him.
  2. If this is a configuration difference, then what is it?
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c # email smtpclient smtp
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1 answer

5.7.1 Unable to send - this means that the message was addressed to a user in another domain, and the mail server could not "relay" it (basically, it cannot send it to another domain

The second error is "mailbox unavailable" - this means that the server determined that the domain belongs to the server, and it determined that the mailbox does not exist

So, this is similar to the first case, it is a different domain, and in the second case it is the same domain.

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