I have been blessed to work with a fine bunch on IT professionals over the years – none that I hold in higher regard than members of my current team.

The other day member of my team whom has expressed a firm interest in learning Exchange server in more depth came to me with a set of questions. The first question that he asked me was – how relevant the Exchange MTA service is considering that we have no legacy x400 requirements (I’ll get shot for called x400 “legacy”).

He had noted that the MTA service was still functional on our Exchange clusters and was curious why.

I thought that this was a good question, which although I have seen covered elsewhere on the Internet I thought that I would have my own stab at an answer

In this article I would like to go through the answer that I have given to Anil and how depending on the version of Exchange you are running affects your MTA requirements.

What is the Exchange MTA (Message Transfer Agent)?

The Exchange MTA has been a major part of Exchange for a number of years – certainly up until Exchange 2003. It is primarily responsible for the transportation of non SMTP based messages within an Exchange environment.

You will hear a lot about the MTA being the key part of transporting e-mail to and from X400 compliant messaging systems (such as Domino, GroupWise and Notes via associated connectors).

If you have an environment which is co-existing between Exchange 5.5 and Exchange 200x (except 2007) the MTA will also handle all RPC traffic between the 5.5 and 200x servers.

One common misconception that I have come across over the years is the belief that the MTA Service (or Microsoft Exchange MTA Stacks as it is known) in Exchange 200x plays a part in all forms of message routing – this is not true.

Protocols such as SMTP have their own internal routing functions which are independent of the MTA functions.

There is a huge amount of information around on the Internet about the MTA service – however if you are really interested in a firm technical description, the best resource that I have found is located here: http://technet.microsoft.com/en-us/library/aa997788(EXCHG.65).aspx

Ok – but what about the question – why are you running X400 if you don’t need it?

Good question (and one that if you search the Internet is answered many times), however I would like to elaborate.

I am old school Exchange, I started out with Exchange 4.0 and then progressed through to Exchange 5.5 – all of which had a very high dependency on the MTA Stacks service (you would not consider disabling it back then unless you wished to have problems).

Exchange 2000 was released, and again the story was that even if you did not have a requirement for x400 transport or Exchange 5.5 servers in your organisation you should still keep the MTA Stacks services running – Microsoft have explained this stance for “supportability” reasons as “unexplained” problems could occur within Exchange 2000 if the MTA service was not running.

Now most articles that I have read on this subject are fairly “vague” on the issues that can occur as a result of the MTA stacks service being disabled within Exchange 2000. The reason that I was given (by an unnamed Microsoft person) as to why there are no definitive answers as to what could happen is that MS is unsure as the to exact effects as it is not a fully tested scenario, however the following are definite problems that will manifest:

• X400 transport is not available
• 5.5 Co-existence models are severely hampered
• Move Mailbox tasks can fail (I am assuming that this would be between a 5.5 and Exchange 2000 site)
• Internal message transport can be disrupted

 

Given the importance of the MTA service in previous versions of Exchange, and indeed considering the time that Exchange 2000 was released I think that one could not blame Microsoft for not recommending disabling the MTA service. If you consider the following:

• Microsoft had a product in the market that relied on MTA in 5.5
• Microsoft released a successor to 5.5 and needed to entertain migration scenarios to the new 2000 platform from the 5.5
• The ground that Exchange had in the market compared to other competitors meant the if they were to tempt people away from Notes for example they needed to maintain x400

 

In Exchange 2003 things changed – essentially due to changes in the functional architecture of Exchange you could disable the MTA service as long as the following scenarios were met:

• Your Exchange 2003 Server routes messages to a 5.5 Org
• Your Exchange server is making use of connectors to x400 based mail systems
• Your Exchange server is directly interfacing with x400 based mail systems

 

If you are considering disabling the MTA stack services on your Exchange 2003 installation as you feel that you meet all of the criteria as per above I strongly recommend that you have a read of:

http://support.microsoft.com/kb/810489

The article takes you through how to accomplish disabling the MTA Stacks service but before you begin I would urge you to think about the following:

• Why are you disabling the MTA service?
• Is is because you wish to reduce your attack surface from exploits?
• Is it because you need to reduce the amount of resource usage?

 

If you are reducing your attack surface – I would urge you to balance risk over benefit - for example – In your current company you may use Exchange clusters and comply with all the scenarios above to warrant disabling the MTA stacks – but as the KB article explains removing the MTA stacks service from the first EVS is a simple task to accomplish – however if your company was to then acquire another which uses Notes for example – you have a major issue in establishing x400 based communication again.

If you are looking to disable MTA stacks from a resource point of view and you comply with the given scenarios which warrant disabling the service – then I would argue the amount of resource saved would be negligible as the service was never used to a degree where it would contribute to the utilisation of the system – if you need to claw back resource in such a manner you might need to review the whole of you installation.

I hope that you find this useful.