I have recently discovered the Raspberry Pi, and I have a question. I did not find any answer. Can I configure one Raspberry Pi 2 B or 3 B to run 2 servers at the same time (for example a web server Apache and a mail or voip server like mumble)?
The short answer is yes. The longer answer is that the Pi is resource limited (1GB of ram and a processor that is approximately the same speed as a 2010 desktop). So the more things you ask the system to do the slower and more unstable the system will become. At some point the performance will become unacceptable. The only way to tell for sure is to test it and determine for your particular use case (number of users, voip calls, emails sent/received) if the throughput and performance is satisfactory. In a worst case scenario you could split the load across multiple Pi's (and likely spend less than a new desktop).