Multiple Mail Server Types and using a Mail Transfer Agent

Currently, I'm using SmarterMail 5.5 Enterprise as an email server for multiple domains (~30 total users).  My situation is that I need exchange-like behavior for 2 or 3 users.  And, by exchange-like behavior, I mean syncing of email, contacts, and calendar for Outlook 2007, Apple Mail, and iPhone.  For these 2 or 3 users, I've narrowed it down to the following 3 options:

1.  SmarterMail 6.x Exterprise with ActiveSync Add-on (would cost $200/year, but could add up to 25 user accounts)
2.  Exchange 2010 Standard (would require the purchase of Windows Server Standard, Enterprise, or Datacenter, because I currently only own licenses for the Web edition.)
3.  Google Apps Premier

The benefit of option 1 is that I wouldn't need an MTA/email that's the option that I'm leaning towards.  However, I've read in several articles indicating that utilizing an MTA is a best practice.  So here are my questions:

1.  Any advice/suggestions (not limited to the options I mentioned)?
2.  What software should I use for the MTA (can be windows or linux)?  Is it possible to configure user-specific email routing at the MTA level?
3.  Even if I go with SmarterMail, should I stand up an MTA anyways?  Why?
4.  Any suggestions for redundancy/failover in case the email server goes down?

I'm open to any Linux and/or Windows solution, as I'm planning to virtualize a couple of servers.  Thanks in advance.
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All of the above solutions are considered "MTA"s, they will all send and receive mail without the aid of any other software/servers. You can use another MTA in conjunction with any of them if you want, but it's not really necessary.

You should also look into Zimbra. There is an open sourced version that is free to use and has all the features (I believe) that you're looking for. It runs on linux with is also free, so there would be no cost to you at all. There is also a supported version you can buy.

As for redundancy/failover, it all depends on what you can afford and what you can live with. If you can live with small amounts of possible downtime (like while you restore from a backup) then it probably isn't with the cost.

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msdcdevAuthor Commented:
I think I'd only need an MTA if a single email domain had users on different email platforms (i.e. had 2 users on Google Apps and 2 users on my SmarterMail server).  I assume that an MTA could handle this routing, right?

I actually just watched a webinar about zimbra.  It sounded like a good option, but it doesn't look like the open-source version provides the over-the-air exchange-like syncing that I require:

You're probably right about the redundancy/failover thing, but I guess that's a benefit of Google Apps - you can lean on Google's reliability.
All mail servers are MTAs. You would need a special setup if you had users in the same domain space hosted on different platforms. It's possible, but could get pretty ugly.

You could use an additional MTA in front of your email server to do things like spam/virus filtering. I have a Linux Sendmail box in front of my Exchange 2003 server at work and it works great. That way I don't need to install any extra software that gets its fingers into exchange.

If you only have 25 users, I'd suggest you just go with google and let them worry about the servers and uptime. Maybe if you had at least a couple hundred it might be worth doing it yourself.
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msdcdevAuthor Commented:
Sorry, I meant a separate MTA in front of my email server like you mentioned.  That MTA would handle the routing based on user.  Do you know if Sendmail provides DomainKeys and DKIM signing?  If so, a gateway MTA may be a good option regardless.  It may also allow me to move between email platforms without mucking with DNS settings.  I think Google apps already provides DKIM and DomainKeys.  Currently, I'm using a third-party plugin for smartermail to handle it.

Even though I have under 25 users, the problem with moving everyone to google apps is the daily email limit.  Several of our websites send transactional emails, and I don't want that to be an issue.  I guess the other option is to set up an email-specifc domain (kinda like facebook does with, and send the system generated emails through it.  I don't like it that emails would come from a different domain (seems phishy), but I guess it's acceptable if there's no other option.

The cleanest solution still sounds like SmarterMail.  It would cost me $500 up front and $200/yr, but I'd have up to 25 activesync users.  Feel free to argue for Exchange if you think it's a better option, but I don't want to pay for Exchange and Windows CALs for each system email account.  However, I guess CALs are perpetual, so there's no annual fee...correct?

I don't think any of them will have DKIM support built-in, but there are addons you can get for just about any MTA to support DKIM, including sendmail.

I'm still not sure any system will let you divide the namespace based on user to different systems. Typically to do something like that you'll have to rewrite the address like this:  > > >

Then on each system just masquerade as "" when sending. This setup would work great, but as the number of users goes up, it can be a huge pain in the ass.

As for a case for exchange...meh, it does a good job I guess. I'm in education and we get huge discounts on licensing (exchange cals are like $5 and server licenses are like $100 for us) so cost isn't a big deal. And yes once you own the cal, there is no recurring fee.

If you already have smartermail and are familiar with it, it's probably the best way to go.
msdcdevAuthor Commented:
Have you heard anything about Kerio?  I may install the trial just to give it a go.  It's a little more expensive than SmarterMail, but the license is perpetual.  I don't like the idea of an annual licensing fee.
msdcdevAuthor Commented:
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