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Why SMTP Messages Fail

A brief look at greylists and blacklists

Why SMTP Messages Fail
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. For a brief primer on the SMTP Protocol in general, as well as the sub-components (such as PTR, SPF, A, MX and MTA records), have a look at The Email School: http://wireload.net/category/the-email-school/

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.

The first step in resolving this issue is to find your public IP address. The easiest way to do this is to go to the following website, 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:

http://whatismyipaddress.com/blacklist-check http://www.mxtoolbox.com/blacklists.aspx http://www.senderbase.org/home

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.

If you find that your IP address is listed, you have a few options:

1) Attempt to delist your specific IP address from the blacklists by notifying them of the problem.

2) 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.

3) 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. Earlier, we mentioned that your IP being listed as a result of your ISP's IP range being untrustworthy, is only one possibility for your IP being listed with the blacklists; 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, this may not be the end of the process. 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 being listed, 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 also important to note that if you repeatedly submit delisting requests to the blacklists without resolving the issues that caused your IP address to be listed in the first place, you will be permanently blacklisted and can never submit another delisting request for that IP address without going through a very long, intensive and cumbersome process involving contacting the lists directly. It is very important that you not overuse the delisting request function.

There are some notable exceptions to this, however. 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 and what they have or have not redirected to your own SMTP server, if you have one.

In any event, 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:

1) 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.

2) 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. If they do take action to resolve the issue, you should be able to resume using direct delivery for your SMTP messages. This process, however, can sometimes take months to resolve which leads us to the final option: Change your method of delivery to relay through an SMTP server.

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 ISP's 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 in order to use this method of delivery:

1) What SMTP server do you have access to?

2) Does that server require authentication? If so, does it support LOGIN, PLAIN or CRAM-MD5 as methods of authentication?

3) What specific authentication credentials should you use to authenticate your SMTP traffic?

Once you get those three questions answered, you can then reconfigure SMTP for relayed delivery instead of direct delivery.

If, however, you don't have an SMTP server available to you, you can't have your IP address delisted from the blacklists and your ISP isn't willing to work towards resolving the larger issue with the MTA, there is a last resort option. You can use a third party mail relay service, such as DNS Exit, 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 ISP's. The drawback to this is that third party mail relay services do charge fees to use their services and the fees are often based on the number of messages you need to send.

About the Author -
Sharon Housley is the VP of Marketing for NotePage, Inc. a software company for communication software solutions. http://www.notepage.net

System Requirements  
  Processor   Pentium 500 mhz or better
  Memory  128 Mb of RAM
(typically 256 Mb total memory for Win 2000, XP, 2003, or 1 Gig for Vista)
  Hard Drive   50 Mb Hard Drive Free (for application & database)
  Connectivity 300 baud or faster modem for dialup connections
serial cable for direct connections or a
dedicated Internet connection
 Operating System   Windows 2000, XP, 2003, Vista (32 bit)



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