Can an older Android phone using Android 4.4.2 act as a spam server?
This is absolutely possible for an Android phone to act as a spam server. There are plenty of security flaws in Android that make this a possibility, and creating a program / script to send spam is trivial.  There is no complete solution, but there are any number of AV programs (available on play store) that could check the phone for virii.

That said, it's more likely the IP has been classified as part of a client/home user range, and has been blacklisted for that reason - ie nothing you have done, or that another device was compromised. If the network is highly secured then it should not matter what device is on it - SMTP should be blocked to the outside world - except through an appropriate gateway.