mocking in go. Is there an easy way?

I come from python and I have been looking for a way to write yests in go. I have come across a few things on SO but they all seem very cumbersome and verbose for something that shpuld be needed all the time.

I am typing on mobile now, will add code later if needed...but for example...

say i have a function that calls smtp.Send spmewhere in the middle. how can I easily test this function?

Say i have another one that hits some outside api (needs mocking) and then takes the response and calls something like ioutil.Readall()...how cpuld i make my way through this test function and mock the call to the api and then pass some fake response data when Readall is called?