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Error 550 When Sending Email

When sending an email, you may receive a failure notification containing the following:

<Third-Party SMTP Host>
#550-Verification failed for <name@domain.com.au>
550-Unrouteable address
550 Sender verify failed

(where name@domain.com.au is your email address.)

If this occurs for all messages, check that domain.com.au is properly registered (if the domain registration has been allowed to lapse, such as by not paying the renewal, it will start to disappear from the Internet), and that it is configured with a proper MX (mail exchanger) record. A simple check that the domain is still operational is to check your website (www.domain.com.au). To check the MX record, open a command prompt and type:

nslookup
set type=MX
domain.com.au
exit

If no MX record exists, the domain needs to be reconfigured. If an MX record exists, check that it is correct.

If the error occurs only for a certain subset of receivers, this may be because your domain (domain.com.au) has recently changed its name servers or at least its mail exchanger, and the previous name server or mail exchanger was hosted by the same ISP hosting the domains of the receivers you cannot send to.

For example, suppose you are sending to receiver@domain2.com.au. This domain's mail exchanger is set to a particular ISP, and the email flows to that ISP. Once received, the ISP checks the validity of the sender (name@domain.com.au) by looking for a valid MX record on domain.com.au. This is primarily an antispam mechanism, to block messages with ficticious reply addresses.

However, instead of querying the global DNS to obtain the domain's name servers (and subsequently the MX record), the ISP checks its own name servers. Because the domain was recently hosted by the same ISP, it finds a full set of records against the domain, including MX, which is set to itself. The ISP then performs a further antispam check by verifying the sender against its own mail servers, but it does not exist. It therefore rejects the message and returns it to the sender. (Which is somewhat paradoxical, since it has just deduced that the sender doesn't exist…)

To determine if this is the case, check the records held by the ISP. First, determine the name servers used by that ISP, say ns1.isp.net.au.

Open a command prompt and type:

nslookup
server ns1.isp.net.au
set type=MX
domain.com.au
exit

The records returned by this command should be the same as those produced by the commands as above (which by default return the domain records held by your network provider). If they are different, the ISP has incorrect records.

If this is the issue, contact the ISP to ensure that your hosting plan is fully deleted and the DNS entries are removed from their name servers. Or, if possible, first alter the entries held by the ISP so that they are correct, and then have it deleted.

NB. Modifying domain records should only be performed with care and preferably by your IT support provider. Do not contact your hosting providers with instructions unless you are absolutely certain you know what you are doing.

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