There is a reason on why some Internet service provider such asTelekom Malaysia Bhd. and Telstra which is the broadband product creator of BigPond from Australia blocked the port 25which is widely use on sending out emails which is created onClient email application such as Microsoft Outlook, Mozilla Thunderbird, Apple Mac Mail and etc. There is a reason whereby this blocking was implemented which it is to help on fighting to reduce the spam emails that was send everyday to our mailbox.
Hereby below is an officially email which Telekom Malaysia Bhd.provided to all its customer:
"Dear Valued Customer,
Telekom Malaysia Berhad wishes to inform its customers about TM's initiative in combating spam in its Internet Protocol (IP) network to improve our quality of service.
Recently, many anti-spam organizations have blacklisted a large number of IP addresses from TM's network. Due to this many customers have been unable to send emails from their mail server to companies who might be using database from the abovementioned anti-spam organizations.
These spamming activities by a small group have affected a large number of our customers, regardless of the nature of their Internet usage.
Therefore, TM is taking immediate action to address this issue. Effective 3 December 2007, TM will block OUTBOUND Simple Mail Transfer Protocol (SMTP) traffic or port 25 for all out going e-mails from dynamic IP addresses. Only Outbound SMTP traffic from smtp.streamyx.com and smtp.tm.net.my will be allowed.
With this implementation, Streamyx customers who have their own mail server will not be able to send e-mails. As an alternative, TM is providing an open relay server for these customers who use dynamic IP addresses. Kindly refer to http://webmail.tm.net.my/info/proxy.html on how to configure your email client in order to utilize the relay server. Alternatively, your e-mail aggdministrators can refer to http://webmail.tm.net.my/info/smtp-proxy.html as a guide to configure your company's email servers.
Please be assured that TM is proactively taking all possible measures to ensure that these spamming activities do not reoccur in the future. We seek your kind co-operation to implement the necessary security measures to protect your computers that are connected to the Internet from any e-mail abuse, virus infection, spyware and malicious code.
For further enquiries and assistance, email us at email@example.com or call TM at 100 and select 'Internet Services'."
On the other hand which is referring to Australia Telstra BigPondbroadband services which also implemented the same blocking with some others provider was also doing the same implementation at Aussies Internet services as well.
You may refer to below link for the article on where and how to change the port 25 to another available port for the Client email application available: