Question

I am getting “Mailbox unavailable. The server response was: 5.4.1 Unable to relay for [e-mail]” when I try to send the mail using C# code.

Then I tested my smtp server here.

Here when I enter my smtp server, sender and recipient name. I got followoing message:

MAIL FROM: [e-mail]

SMTP -> FROM SERVER:

250 2.1.0 Sender OK

RCPT TO: [e-mail]

SMTP -> FROM SERVER:

550 5.4.1 Relay Access Denied

SMTP -> ERROR: RCPT not accepted from server: 550 5.4.1 Relay Access
Denied

Message sending failed.

Answer 1

That’s a config issue in the mail servier. It doesn’t allow you (your IP, your sender address) to send mail to the recipient.

A properly configured mail server is extremely restrictive with relaying, to prevent it from being abused by spammers to send mails.