Problem: Can't send email to external addresses

Causes and Resolutions

There are multiple possible reasons why you may be unable to send messages to external email addresses.

Cause 1 - Outgoing connections to port 25 blocked by a firewall

The first and easiest thing to check is to make sure a firewall is not blocking outgoing connections on port 25.  To test this, go to a command prompt on your pc or server and try to telnet to a few external mail servers.  You can telnet to our A1Webhosts mail server on the SMTP port (25) by typing the following:

    * telnet mail.a1webhosts.co.uk 25

You should get a response like this:

220 mail.a1webhosts.co.uk

If you get a response starting with '220' you can just type 'quit' to end the session.  If the connection fails then you likely have outgoing connections on port 25 blocked by your firewall.  You will probably want to try to connect to one more external mail server to be certain this is the case.

Cause 2 - The internet connection is down.

The next easiest thing to check is that the internet connection is ok.  You can check this by simply opening up Internet Explorer and trying to browse a few sites.

Cause 3 - Bad DNS Setup

If the internet connection is ok and outgoing connections to port 25 aren't getting blocked by a firewall, the next most likely culprit is the DNS settings.  To verify that your DNS servers are returning correct information for MX lookups, go to a command prompt and type the following :-

    * nslookup
    * set type=mx

Now type a domain name to retrieve it's MX records.  When I look up the MX records for A1Webhosts.co.uk I receive the following:

a1webhosts.co.uk            MX preference=10, mail exchanger = mail.a1webhosts.co.uk
mail.a1webhosts.co.uk      internet address = 78.32.11.55

If your DNS servers are not returning correct (or any) MX record information you will need to fix them before you will be able to send out email correctly.


Cause 4 - Mail rejected due to host name or invalid reverse DNS entry

If the problem is only with sending email to certain domains, there are a couple of settings that may affect this.  The first is the host name of the server.  This should be set to a fully qualified domain name.  We use mail.a1itsolutions.com for ours.  The other thing to check is that the IP that the server uses to send outgoing mail with has a valid reverse DNS entry.  Note that this is the external ip address, normally the routers publicly assigned one.

Cause 5 - Your mail server is listed by a Spam RBL list

Another reason mail servers may reject mail from your mail server is that it is listed on a spam RBL list.  There are many spam RBL lists on the internet that keep track of servers reported to send out spam or have been verified as an open relay.  To see if your servers ip is listed, do a search for sites that allow you to check an ip against multiple RBL lists.  You may want to just go to a search engine and type in something like 'multiple spam RBL list' or something along those lines.  At the time of this writing I use http://rbls.org/  or http://www.dnsstuff.com/  to do multiple RBL lookups for a mail server.

Cause 6 - Something else

If you are still having trouble sending messages, please contact A1 IT Solutions Support for help with this issue.