SMTP Vs. elastic_email
First of all, I'm assuming that you are referring to the SMTP Authentication Support and the Elastic Email modules. The answer to your question is rather straightforward: you only have to use one of the 2 modules. So, if you are sending all your emails using the Elastic Email module, you can pretty much disable the SMTP Authentication Support. 

The reason is that the 2 modules are actually doing the exact same thing: they give you a way of bypassing the default PHP mail() function. If you have decided that you want to use Elastic Email, you don't need the SMTP Authentication Support module (and I'm not sure how they interact with each other so, it's better if you disable SMTP just to be on the safe side on this).