David.M.Clark 1961 Page

March 17, 2018, 10:13 pm

My How-To Articles

Article Title Telstra NBN blocking outbound SMTP
Article Reference 16100601


Running your own e-mail server with its own hosted e-mail domains behind a Telstra NBN connection with a fixed/static IP address. There is a need to use a smart of relay host from your own server which is traditionally the ISPs SMTP server(s). This connection is a home Internet connection.


The outbound SMTP servers at the ISP level do now allow your SMTP based e-mail server to send e-mail, ie., relay through their SMTP servers to external e-mail addresses. This only happens if the NBN external IP address for your connection is a fixed/static IP address. If you remain on the NBN dynamic IP address setup, sending via the ISPs SMTP servers works correctly.

At the start the NBN connection was using a dynamic IP address and the SMTP outbound traffic continued to be sent via mail.bigpond.com, as it had done since the early to mid 2000s via a BigPond cable connection, without any issue.

Once connected to the NBN using a fixed IP address, all outbound SMTP traffic bounced or came back as relaying denied.

Upon reporting this issue, which was elevated to the Platinum service, all that could be offered was the connection be assigned an new fixed IP address.

While the change to the fixed IP was cutting over to a different fixed IP address, outbound e-mail started to work as the IP address for a brief time was dynamic.

Once the new fixed IP address was in place, SMTP outbound traffic was again not possible.


For a home based connection, unlike a business connection, SMTP outbound is only allowed using SMTP Authentication. This was suggested by a Telstra e-mail tech which I have now confirmed is the case in my setup.


For SendMail SMTP Auth on Linux, I needed to create the /etc/mail/authinfo file with the following content:

AuthInfo:bigpond.com "U:your_name@bigpond.com" "P:your_password" "M:PLAIN"
AuthInfo: "U:your_name@bigpond.com" "P:your_password" "M:PLAIN"

Where your_name and your_password are the username and password issued to you by Telstra NBN for your NBN account.

Next you need to add a line to your /etc/mail/sendmail.mc file below the confTO_IDENT line, so:

define(`confTO_IDENT', `0')dnl


define(`confTO_IDENT', `0')dnl
FEATURE(`authinfo',`hash -o /etc/mail/authinfo.db')dnl

And finally run:

m4 /etc/mail/sendmail.mc >/etc/mail/sendmail.cf
service sendmail restart


systemctl restart sendmail

(c) 2018 David.M.Clark 1961