I'm building an app, where people can work with his team and they need an easy way to send messages each other.<p>It would be a good idea to setup an "internal" smtp server to send messages between them? Then I could use a lot of existing tools around that (smtp libs for an mobile app for example...)
If it should be "simple" and internal, no it wouldn't fit well. I wouldn't recommend it for any type of in-app communication actually. Depending on the architecture of the app you're about to build I pretty sure there are tons of more reliable and scalable solutions.
How about using eXtensible Messaging and Presence Protocol (XMMP, a.k.a. Jabber) instead? That one at least doesn't have all the horrible historical baggage that e-mail is lugging around...because e-mail is anything <i>but</i> simple.