What I think
@cliveren13 is referring to is, if you move from using an iPhone to using an Android device then you have problems.
iMessage works by maintaining a list of known devices in the background. If your friend has sent you an iMessage in the past their device will default to that in the future. If you move to Android then your mobile can no longer receive iMessage but their device may try to reach you that way. You have to ‘de-list‘ with iMessage to fix that.
The problem is worst when you have a device other than your mobile which can still receive iMessage (e.g. an iPad). In this scenario iMessage believes it has successfully delivered the messages but the recipient’s mobile can’t receive it of course.