The answer by @bartłomiej-mucha is true for version 3.5 and above. In older versions of EXM (and ECM) you have to resort to implementing an extension as explained in [Customize the Renderer URL setting in ECM][1]:

> To be able to specify the Renderer URL manually, you can implement a
> small extension.
> 
> Check the following example:
> 
> Create a custom processor to update the Renderer URL with the value
> from a configuration file. For example: 
> 
>     public void Process(PipelineArgs args)
>     {
>       GlobalSettings.RendererUrl = Settings.GetSetting("ECM.RendererUrl");
>     }
> 
> Add this processor to the <initialize> pipeline of the web.config:
> 
>     <pipelines>
>       <initialize>
>         ...
>         <processor type="<your-processor-class-name, your-assembly" />
>       </initialize>
> 
> Specify the required value of the ECM.RendererUrl setting in the
> web.config. This address should be accessible from the ECM server
> (where the message is being created and dispatched) of your Sitecore
> environment:
> 
>     <settings>
>       ...
>       <setting name="ECM.RendererUrl" value="http://yourhost" />


  [1]: https://kb.sitecore.net/articles/837879