Mailbox unavailable The server response was 5 7 1 Unable to relay Error

Question

I have Hosted one of my website on netsol server  From there a contact aspx has to send email using exchange server  When I attempt to send an email      Error  Mailbox unavailable  The server response was  5 7 1 Unable to relay   Code   MailMessage message   new MailMessage   xxx example com                                            test user yahoo com                                           New Message                                           Exchange    SmtpClient mail   new SmtpClient  exchange abc com   29   mail EnableSsl   true  mail Credentials   new NetworkCredential   xxx example com     password    mail UseDefaultCredentials   false  mail DeliveryMethod   SmtpDeliveryMethod Network  mail Send message     Options I tried    Tested on Port 465 or 587 or 25  Changed SmtpDeliveryMethod PickupDirectoryFromIis can t configure IIS  SMTP server  as it is hosted on someone else s server

User · Answer

I use Windows Server 2012 for hosting for a long time and it just stop working after a more than years without any problem. My solution was to add public IP address of the server to list of relays and enabled Windows Integrated Authentication.

I just made two changes and I don't which help.

Go to IIS 6 Manager

Select properties of SMTP server

On tab Access, select Relays

Add your public IP address

Close the dialog and on the same tab click to Authentication button.

Add Integrated Windows Authentication

Maybe some step is not needed, but it works.

User · Answer

WE had this issue  everything was setup fine in terms of permissions and security   after MUCH needling around in the haystack  the issue was some sort of heuristics  in the email body   anytime a certain email address was listed  we would get the above error message from our exchange server   it took 2 days of crazy testing and hair pulling to find this   so if you have checked everything out  try changing the email body to only the word  test   If after that  your email goes out fine  you are having some sort of spam heuristic filter issue like we were

User · Answer

The default configuration of most SMTP servers is not to relay from an untrusted source to outside domains  For example  imagine that you contact the SMTP server for foo com and ask it to send a message to bar xyzzy com  Because the SMTP server doesn t really know who you are  it will refuse to relay the message  If the server did do that for you  it would be considered an open relay  which is how spammers often do their thing   If you contact the foo com mail server and ask it to send mail to somebody foo com  it might let you do it  It depends on if they trust that you re who you say you are  Often  the server will try to do a reverse DNS lookup  and refuse to send mail if the IP you re sending from doesn t match the IP address of the MX record in DNS  So if you say that you re the bar com mail server but your IP address doesn t match the MX record for bar com  then it will refuse to deliver the message   You ll need to talk to the administrator of that SMTP server to get the authentication information so that it will allow relay for you  You ll need to present those credentials when you contact the SMTP server  Usually it s either a user name password  or it can use Windows permissions  Depends on the server and how it s configured   See Unable to send emails to external domain using SMTP for an example of how to send the credentials

[c#] Mailbox unavailable. The server response was: 5.7.1 Unable to relay Error

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