Sure, you can run one, good luck getting even a halfway decent delivery rate to mailboxes at any major mail provider. Even if they never receive a spam message from your server, your server is an "unknown" which counts against you. And if one person in your small company of 10 or 100 or even 1000 people gets their e-mail hacked and sends spam? Prepare for the rest of them to get punished for it. Running an SMTP server is a nightmare which is why, over time, more and more of the economy has just shifted their SMTP servers to organizations who professionally run SMTP servers instead of having their own.