State owned telco and ISP MTNL is blocking a part of its broadband network that allows sending of emails through private email servers after ISPs in Mumbai came under a heavy DDoS attack last week, crippling internet speeds in the city. MTNL will be blocking ‘Port 25’ used for sending emails on private servers, which will essentially stop spammers and hackers from sending out bulk emails in a network using hijacked PCs.

MTNL has circulated emails to its broadband users; one of our readers had pointed it out to us in a tweet:

For example, I could send and receive mails from my MediaNama mailbox using a standalone Thunderbird email client, and if the mails are routed through Port 25, they will not be sent to the recipient, because of the ISP block on Port 25. Note that I will still be able to receive emails in my private email server, since a different port is used for that.

MTNL mentions that some of its users may not have installed an anti virus/spam filtering software, allowing hackers to take control of the PC and carry out DDoS or spam attacks that can clog the entire broadband network.

Corporate users have private email servers registered under their domain name, and some of them may not be send and receive emails. MTNL is asking such users to instead use their own private EMTP email servers, by manually configuring email clients like Thunderbird to route email through “”.

However, a user on Broadbandforum points out that MTNNL’s own sever uses Port 25 for SMTP mail transfers and is prompted with a warning “about their certificate”.


ISPs have blocked Port 25 in the past

Note that, MTNL isn’t the only ISP doing this in India, to keep a check on spam. Elsewhere in the US, ISPs including Comcast and Time Warner have blocked Port 25 to keep a check on spam attacks. Another user on using BSNL broadband wasn’t able to send messages via private server using Port 25; the post was dated July 2011:


I’m subscribed to a BSNL connection, and seems like my ISP has enforced Port 25 block already:


To check if your ISP has blocked Port 25, run CMD and type out the following command “telnet 25”. You can replace the address with your own or other email server address.

MediaNama’s take
Blocking Port 25 means blocking a part of the Internet that is essentially responsible for sending and receiving emails on private servers. Although MTNL has informed users about this in advance, it should be providing a workaround to disable such blocks for users who depend on private servers for email. We are not sure if MTNL has enabled this option; MediaNama has written to MTNL for a comment. MTNL’s Support Team said “Please use the SMTP port as 587”. Customers will have to manually configure their email client to send and receive emails through Port no 587 and route it via MTNL’s email server. More on that here.

Full Text of the MTNL notification to its users:

Dear MTNL Broadband User:

You may be aware of the DDOS attacks that crippled lot of ISP’s in Maharashtra last week. While the MTNL infrastructure was not affected by this, we are taking some additional measures to protect broadband users. There are many broadband users who do not have effective Anti Virus systems. Hackers attack these systems, install zombies and then carry out

-SPAM attacks (frequently), and
-DOS attacks (infrequently).

To stop SPAM attacks, MTNL Mumbai will be blocking port 25 for all broadband users. Port 25 is typically used by zombie PC’s to send out SPAM. These attacks happen without the knowledge of end users and make the Internet connection slow and increase the broadband user’s Internet usage. By blocking this port, MTNL will be stopping these attacks.

This change should not affect the typical user, as Internet and browsing access does not require port 25 access. Corporate email users can ask their System administrators to use a different port (587, 465, etc.) and continue to use their emails without any interruptions.

This does not affect GSM / leased line users. We expect very few users to be affected by this. If you are affected by this, as a short term measure, you can use relay services from MTNL. We have provided the following resources if you have any queries:

Guide on how to configure MTNL SMTP relay services
Support Email:
Phone: 022-24331175

This measure will be implemented on 28-7-2016 evening. Configuration changes (if guide is followed) should be done by users on or after 29-7-2016.

Thanks & Regards
MTNL helpdesk.