How to work with Outgoing SMTP Port 25 Blocking with InterMedia Email

This article does not cover general email technical knowledge such as the differnce between POP3 accounts and an Exchange email account, or the general topic of "how to setup your email program to use an InterMedia email account." For that last one, at least, refer to this FAQ item (how-to-setup-intermedia-email-in-your-email-program<<<<< Link ) which explains all of the necessary incoming, ougoing and authentication settings necessary to send and receive email.

This instructional topic focuses on one specific issue which can sometimes confuse a user and cause issues when attempting to setup email in their email application such as Outlook or Thunderbird, or on their phones/wireless devices.

The Issue - Outgoing SMTP Port Number is Blocked

The issue with Outgoing SMTP restriction that is increasingly used as a method for Internet Service Providers (ISPs) to improve email security over their network.

The Symptoms:
In most cases, when there is an issue with the Outgoing SMTP with an ISP, the user will have followed all instructions for setup of their email application, and can properly receive email always. However, when sending out email, they are able to either send out sporadically (such as "one email goes out every few hours but then second and third emails fail"), or not at all.

The Background/basics:
SMTP (Simple Mail Transfer Protocol most often using a default TCP port 25) is the network mechanism for sending email messages. Illicit connections to computers through port 25 (SMTP) can be a security risk. This port is frequently misused to:

- Send spam, particularly spam with illegal or "malware" content

- Gain access to and take control of computers (servers)

Blocking these connections at the network level protects their servers and network from unwanted activities. This restricts users from sending spam mailings from the ISP's network using a different outgoing mail server than that of the ISP. Even though the mail is being sent through a different mail server than an ISP's, the mail header information still shows the messages as originating from the ISP's domain. In order to block these misuses ISPs are increasingly taking the simple step of blocking connections to port 25 by default at the network gateway for all email that IS NOT relayed over their specific SMTP. 

Please note that the element of port 25 blocking most often affects custom/corporate email, and not the free email that they provide the user as a customer of their ISP. This means that the provided simple, personal email from the ISP such as person1234@bellsouth. net "going out" over the outgoing SMTP of WILL be able to "go out" through port 25. However, relay of any other domain's email for your corporate email such as yourname@companydomain. com "going out" over an outgoing SMTP of WILL FAIL going out through port 25.

The Solution (most common):
In MOST cases, the solution is to use one of the standard alternate ports that InterMedia allows for. The main alternate port is port 1025. The user enters this as the outgoing port number for the outgoing SMTP. NOTE, make the change in the outgoing only... the incoming SMTP port remains unchanged.
In a small number of cases, the ISP simply will not allow a custom SMTP to be used and they instead require the user to keep the provider's SMTP (such as and their instructed outgoing port.


  • Keep in mind that the port 25 block has no involvement with access to email through the "web mail" system. When testing the setup of your email account, check whether you can log in and successfully send/receive email through the web-based email website. When successful, this will help determine that there are issues at the ISP level that need to be resolved.
  • Keep in mind that the preferences/restrictions of the ISP are specific to that ISP only! If a home location ISP seems to be blocked or have issues, try taking the laptop to a coffee shop or public wifi location to test outgoing mail. Some clients that travel with laptops between "home" and "office" need one port for office and another for home!
  • If all of the settings are correct, and the user has tried both of the solution options listed above, then the next course of action is to call the technical support of the ISP for instructions.


  • Currently, the following ISPs are the reported as blocking port 25: AT&T Adelphia Juno NetZero SBC Frontier MSN EarthLink Verizon AOL MediaOne FlashNet Comcast
  • Keep in mind that an ISP may decide to adopt port 25 blocking at any time! If nothing has changed in your email application, and suddenly you are not able to send mail reliably but are still able to receive, then perhaps the ISP has started port 25 blocking and you must make changes in your email application settings to accommodate those changes!
  • Unfortunately, no rules and guidelines are universal! There are some ISPs, such as AOL which simply do not allow the relay of "non" email to go out over their service. This means that users with AOL service can ONLY use webmail for sending mail (or preferably consider switching to a normal ISP).
  • On occasion, we have found that some ISPs do not allow the alternate port's that may be available for use from InterMedia, and instead provide their own special/alternate port for sending out mail. In these cases, it is often only a phone call with a live person at the ISP's tech support that will uncover the situation and solution quickly.

Interesting resource articles for further reading on the topic:

Optional Services for Consideration:

No-IP’s Alternate Port SMTP service
$19.95 / month
How it Works - "ISPs often block port 25, the port on which email is sent. This is done to prevent spam from unwitting or compromised computers. But this also blocks many legitimate users who would like to send email from their personalized accounts. Other spam stopping techniques, such as requiring reverse DNS also hinder legitimate users who want to send email. No-IP's Alternate-port SMTP gets around these problems by letting you inject mail to our system for delivery. We take care of the details and you just send. Not only that, when using Internet connections provided at airports, hotels, friend's places, even at work, you'll never have to change your SMTP settings!" -