John Howard - Senior Program Manager in the Hyper-V team at Microsoft

Senior Program Manager, Hyper-V team, Windows Core Operating System Division.


Resolving SMTP error 550 5.7.1 Unable to relay for Error 0x800CCC79

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I was struggling with this error message today - everything I had found on the Internet suggested it related to security. Well, it does, of sorts, and a careful look at the configuration of your SMTP server without mis-interpreting a dialog box.

If you open the properties for your SMTP server under IIS admin, select the Access Tab, and at the bottom, click "Relay", you will see a check-box at the bottom, which is by default selected, saying "Allow all computers which successfully authenticate to relay, regardless of the list above". In my head-scratching to work out the cause of the error, I had mis-read the message as "Allow all users which succ....". The answer therefore was simple, above that check-box on the relay restrictions tab, there is a options dialog which by default Only the list may relay. Simple as adding your server to the list, or selecting All except the list below. The first one is obviously more secure.


  • not only "more secure", it keeps you from being an open relay! Unless you enjoy sending out millions of emails on behalf of spammers, DON'T use the "all except the list below" option.


    Of course this isn't of as much concern if TCP 25 (SMTP) on your mail server isn't open to the Internet. I'd still never setup a completely open relay, Internet-accessible or not.

  • Thanks a lot... I have been trying to work on this problem since many many days and never thought the solution will be so simple! Thanks a lot :)


  • I have two servers internal and ext(front end and backend).Which server I shud add to the relay list my internal server or my external.

    I  am working on my internal right now..pls let us know which entry to add

  • Hello,

    I have the same problem with a single domain name that my users and servers seem to be sharing: the inablility to send to a specific domain. We get 5.7.1. Unable to relay ... or NDRs stating the address is unroutable. I have tried to telnet test to my servers and get 5.7.1. ...  I have tried the configuration described above with no luck. I know it's a valid email address as some of my people can email to the name from external accounts. Any ideas?

  • thx for sharing the solution, it worked for me

  • If you are using the pickup directory on a Windows Server 2008, and nothing sends, try turning off UAC (User Account Control). That worked for us.

  • Thank you, you solved my problem ...

  • I was trying to fix this problem since long back. Thank for the solution.

  • The link Jenny has posted is a virus containing an IFRAME exploit....I would advise the link and comment should be removed.

  • Removed! Thanks, John.

  • Good Day to all, i am programming a php file that could send data/info to our perspective recipients but this error occur:

    220 ethnic.ppc.loc Microsoft ESMTP MAIL Service, Version: 6.0.3790.3959 ready at Wed, 6 Nov 2013 09:37:02 +0800

    250 ethnic.ppc.loc Hello []

    250 2.1.0 OK

    550 5.7.1 Unable to relay for

    Ran into problems sending Mail. Response: 550 5.7.1 Unable to relay for - 8

    We already tried to send data with the email "" in our microsoft outlook and it works. Please help.

  • Sorry, no idea. The post was some 8 years back and I'm not authoritative on exchange issues. I would start with the TechNet forums for Exchange.

  • Hello, I too faced SMTP error 550 5.7.1 couple of times but managed to overcome. However gone through an article which discuss about all possible 550 errors related to SMTP..

  • Dear John, Thanks for great tip! I got an other error code when trying to send email from Ubuntu server 12.04 by command lines: postfix/smtp[10700]: 0A083140089: to=,[]:25, delay=0.82, delays=0.08/0/0.63/0.1, dsn=5.7.1, status=bounced (host[] said: 501 5.7.1 ... Sender domain must exist (in reply to RCPT TO command)) All information of my email domain ( and email server's IP are correct. Please help to suggest how to solve this error?

  • Dear John, Your idea worked out very well for and i am very grateful for that. At my work we use Oracle 11g DB server which supports an ERP application. SMTP is configured on this server. So once orders are raised or approved (Stock Transfer Requisitions), e-mails are triggered to the respective parties. My situation was that the e-mails were delivered to users who had access to send e-mail outside our network ( but got the following error message for internal e-mail users (xxx@domain.local) : Authorisation ORA-29279:SMTP permanent error: 550 5.7.1 Unable to relay for xxxx@domain.local ORA-0651:at"SYS.UTL_MAIL",line 654 ORA-0651:at"SYS.UTL_MAIL",line 671 ORA-0651:at"DB INSTANCE NAME

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