The Simple Mail Transfer Protocol (SMTP) is the most commonly used internet based protocol for third party SMS delivery. Almost every major carrier in the US and Canada supports this protocol and, as a result, this is the most heavily used protocol. The down side to this protocol is that it is effectively Email, which means that there are a lot of issues that can arise from using it.
There are two primary ways to send SMTP: Direct Delivery and Relayed Delivery.
Different networks handle SMTP and its related ports in different ways. If port 25 is free and clear (i.e. not blocked by any firewalls along the way), you can attempt to use the Direct Delivery method. However, this method of delivery does rely on your Internet Service Provider's settings, and general spam reputation on the internet.
Sometimes, even if port 25 is open, you still can't use the Direct Delivery method for SMTP because your Internet Service Provider's (ISP) Message Transfer Agent (MTA) is considered untrustworthy. While this is the most common cause of direct delivery SMTP failure, it is important to note that this is not the only cause. In any event, when this happens, the ISP's public IP range appears on several blacklists dedicated to providing third party insight in to trustworthy and untrustworthy sources of SMTP traffic. It is also worth noting your ISP's IP range covers anyone on their internet service, not just your network. Any IP address on one of these blacklists is treated as an untrustworthy source and, as a result, any direct connection attempt to a receiving SMTP system, such as those for cell phone carriers that subscribes to these blacklists, will fail.
Finding and Checking Your IP Address
The first step in resolving this issue is to find your public IP address. The easiest way to do this is to go to whatismyipaddress.com from the computer that is sending out the SMTP messages.
Next you will want to find out if your IP address is listed with one of the major blacklists. There are a variety of resources available for this, such as:
Once you have determined what your public IP address is, go to one of these sites and perform a blacklist check on it. In the results of the lookup, you should see which, if any, blacklists your IP address is listed on.
Options If Your IP Address Is Listed
If you find that your IP address is listed, you have a few options:
- Attempt to delist your specific IP address from the blacklists by notifying them of the problem.
- Contact your Internet Service Provider and inform them of the MTA downgrade and resultant blacklisting. Hopefully, your ISP will work with you to resolve their IP range's status as untrustworthy.
- Change your method of delivery to relay through an SMTP server.
Each blacklist has a website associated with them and most have a procedure for delisting specific IP addresses, because false positives can happen. When you perform the blacklist lookup on your IP address, pay attention to the results. Sometimes, if a system or server on your network is infected with a particular type or style of malware, your specific IP will be blacklisted. The lookup pages will provide the information relevant to why your IP was listed.
Once you've determined that your IP address is listed, you can submit a delisting request with the blacklist hosts. These requests generally take a minimum of one business day to process and once the request has been approved, your IP will be removed from the blacklist. However, if your IP was listed because you have a malware infected system on your network and you don't identify and remove the malware, your IP will immediately be listed again. Similarly, if your ISP's IP range is listed, rather than just your specific IP, your IP will immediately be listed again.
The complexity is that there is only so much that you, the end user, can do to resolve the issue as a great deal of this has nothing to do with your network and everything to do with your ISP's standings with the aforementioned blacklists. It is very important that you not overuse the delisting request function.
Notable Exceptions: Comcast and COX Cable
Certain Internet Service Providers, such as Comcast and COX Cable, require that all SMTP traffic from their networks MUST be passed through a Comcast or COX Cable SMTP server for non-business internet connections on their service. Even for some of their business internet services, even if you have your own SMTP Mail server with an associated A, MX, PTR and SPF record, you may never be able to use direct delivery for SMTP due to the restrictions placed on the ports and the general status of the MTA, PTR and SPF records for your ISP.
When Your ISP's IP Range Is Flagged
If your IP address has been listed because your ISP's IP range has been flagged as a potential source of spam, submitting a delisting request won't do you any good and it leaves you with only two options:
- Contact your Internet Service Provider and inform them of the MTA downgrade and resultant blacklisting.
- Change your method of delivery to relay through an SMTP server.
It is possible that your ISP may not be aware of the issues regarding their IP range. You can attempt to show their Tech Support the results of your blacklist lookups and they may or may not take any action to resolve the issue. This process can sometimes take months to resolve, which leads us to the final option: change your method of delivery to relay through an SMTP server.
Using a Third Party Mail Relay Service
If your IP address is listed on the blacklists and you can't get it delisted and your ISP won't work with you to resolve the larger issue with their IP range, your only option is to relay your messages through an SMTP server that you have access to. Most ISPs have an SMTP server that they will grant you access to for using their internet service and most companies have their own SMTP Server available on their intranet. There are three questions you'll need to have answered:
- What SMTP server do you have access to?
- Does that server require authentication? If so, does it support LOGIN, PLAIN or CRAM-MD5 as methods of authentication?
- What specific authentication credentials should you use to authenticate your SMTP traffic?
If you don't have an SMTP server available to you, you can use a third party mail relay service for SMTP delivery. The benefit to this is that most third party mail relay services will listen for SMTP traffic on non-standard SMTP ports, which are typically monitored or blocked by local firewalls and ISPs. The drawback is that third party mail relay services do charge fees, often based on the number of messages you need to send.
