ISP is blocking the outbound SMTP port (25)?

Posted on 2010-11-09
Last Modified: 2012-05-10
Heard that some ISP block smtp port 25, if port 25 port is blocked how can outlook client send out email to their ISP smtp server?
Question by:okamon
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Expert Comment

ID: 34099204
Some ISP's require SSL authentication which by default changes the port from 25. Other ISP's require a simple port change in the mail settings.

Personally, I have heard of ISP's blocking other providers SMTP ports via dns but never block their own. An example is this:

If you have cox internet you smtp address is it works fine on cox network on port 25, but when you take the laptop to another isp like qwest it blocks from resolving via dns.

Expert Comment

ID: 34099225
I feel it may not be port block but "Mail server address " of SMTP will be different, I suggest consult with ISP the alternate SMTP mail server address.....I am sure ISP will have alternate mail server address.

Or alternate option or  way is to bypass your ISP by creating VPN connection if possiable to that country or region and use POP3 and SMTP

Author Comment

ID: 34099387
So you are saying either ISP really block the port 25 and use ssl authentication(which user different port to communicate with their smtp) or they block other providers SMTP ports via dns? correct?

>I have heard of ISP's blocking other providers SMTP ports via dns
So do you think if i use ip address will work?

and how do I test if they block the port?
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Expert Comment

ID: 34099411
I feel ISP never block port 25 but as per different region or country different "mail server address" is assign.
-> To check port block or not you can
telnet "mail server address" 25.
LVL 12

Expert Comment

ID: 34099448
Yes, some ISP's really block port 25 to any but their own outgoing mail servers.

Among other things, this is intended to help reduce spam and virus distribution -- e.g. when somebody's unpatched Windows machine becomes infected by a virus, and starts sending out copies of itself to everyone in their address book.

Some mail hosts offer outgoing mail on the alternate port 587 in addition to 25.  Few, if any, ISP's actively block port 587.

You can test access to port 25 on any particular host using telnet:
 telnet 25

Here's an example of a successful connection (note that we see a confirmation of what machine we've "Connected to"): telnet 25
Connected to
Escape character is '^]'.
220 comcast ESMTP server ready
221 2.0.0 comcast closing connection
Connection to closed by foreign host.

Here's an example of an attempted connection to a machine that has port 25 open, but which cannot be reached from my Comcast client: telnet 25


Author Comment

ID: 34103903
thank you. But one of my questions still not being answered. How do ISP usually block port 25?

1. they block port 25 for everything and use other port number for their own smtp server?
2. port 25 is not blocked, but filtered by DNS? so users only able to communicate with their isp smtp?
and in this case, if I use ip instead of FQDN, will it work?
LVL 12

Accepted Solution

larsrohr earned 300 total points
ID: 34104147
I believe it is usually by firewall -- they can block all port 25 access, but they can allow port 25 to their own smtp server.
I've heard that in some cases, the ISP firewall may be used to intercept any port 25 requests (to anywhere), and redirect them to the ISP's own smtp server.

I don't think DNS changes are generally used, because that would affect access to other ports as well.  DNS affects the entire hostname, not just arbitrary ports.

So, this means that using an IP versus a FQDN will have no effect.  You would need to use either your ISP's smtp server, or find out if your preferred mail server offers alternate ports (587) or other options.

Author Comment

ID: 34106501
Hi larsrohr, thank you so much! need to ask you one more thing, so let's say my ISP block port 25 to all other smtp server, so let's say if I am having problem sending email to other people and I need to troubleshoot using telnet, will I not be able to?
LVL 12

Expert Comment

ID: 34106689
You'll be able to troubleshoot using telnet, just to verify what your ISP is doing.

If they are blocking port 25 to all other smtp servers, you may have to use their smtp server to send out your email.
If you are using Outlook or Outlook Express, for instance, you may have to temporarily modify your outgoing smtp server setting, to use your ISP's smtp server.
New versions of Thunderbird allow you to configure multiple outgoing smtp servers, so you could just add the ISP's smtp server.

If you have a web interface to your email, you could just use that for now, instead of your usual mail program.

Author Comment

ID: 34108293
i am not sure if you are answering my question, i mean, if my isp only allow me to user their smtp, ex: and I need to telnet to 25, will I be able to? I guess not as my isp block port 25 except their own smpt right?
LVL 12

Expert Comment

ID: 34109161
That's right -- you won't be able to "telnet 25".  The connection will hang at "Trying ..." and not complete.


Author Comment

ID: 34128058
thank you so much for your time. i have another question here if you can help

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