My company has ~40 employees, all using POP3 mail accounts on a shared host. They allow up to 200 email boxes, have no limit on the # of outgoing/incoming e-mails/hour, and we've never had really big problems with them (plus, they're REALLY cheap).
However, we suspect that at some point before we reach the 200 email limit we'll need to upgrade ... but this is honestly just a baseless suspicion due to so many other companies our size using Exchange or other mail servers. My question - how should we guage the right time to upgrade, and what should be our criteria for the next solution?
Consider your need for:
If these features appeal to you, you may with to consider moving to an Exchange setup. You don't have to purchase/license Exchange and all the servers, however. There are a handful of good Exchange hosts out there that can really help your bottom line (if you pick the right one for your needs).
Shared/hosted Exchange means that you don't have to worry about:
I've taken onboard the task of integrating Exchange into the SMB, and it's something that needs a bit of research and a good resource. It can be expensive (depending on your budget), and if it's not in your core competency, or I.T. isn't in your company's core competency, it'll be a challenge to convince the CFO/Controller to dish out the cash. You may have to prove out the benefits, or have the users complain loudly enough.
Run the Numbers!
Perhaps run the hosted Exchange scenario through a spreadsheet. Costs of purchasing and running inhouse vs. hosted. Run it for 2, 5 and 10 years. Remember to include your company's expected headcount in those time periods.
Keeping the POP3 Status Quo
If the users aren't screaming for the features of 'Outlook' like they had in their last job, and you can manage the above issues, then perhaps the move to Exchange isn't for your organization yet. Nothing wrong with that!
Even if the answer is already excepted, I'd like to mention IMAP. I don't understand why people still use POP3 these days. I think nowadays most people work on more then one machine (e.g. at home and at work) or read their mails on a mobile device or via a webinterface. Keeping the mails on the server is a good idea when you want to access them from everywhere.
If you want IMAP support, a great webinterface, Calendar, InstantMessage and so on, you should definitely take a look at Google Apps. It's free for up to 50 users and cheap enough if you want more.
The time to consider moving is when you start getting pressure to support more features, or the same mailbox on multiple devices, such as a desktop AND a Blackberry. But if you're not getting pressure to move, then doing so proactively isn't required in my opinion.
Run the numbers and features on Open Source email solutions as well. You don't have anything really integrated with a Microsoft ecosystem so you will not lose out. They may not be the way you want to go or the integration features offered by an exchange solution might outweigh the benefits of OSS but you won't know until you look.
Check with you users regularly, see if there are complaints or issues starting to crop up or if people are beginning to feel limited by what you have. Also take into account your technology strategy, what can you see people doing in 2 or 3 years?