A gopkg url for only rssgoemail is totally a bad idea. Also, looking at the latest typosquatting attack against the Go's ecosystem, I though that avoiding future issues with my code ending in mirrors around the world is a good idea.
I can't do anything, probably, for the currently released version. However, for the future, when I'll upgrade the dependencies, be advised to just go to the official page, and download the code from there. In the end, alla the dependencies are vendored and there is no real need to use "go install" just for this.
If you do not know the repository URL, here we go:
https://lab.abiscuola.com/rssgoemail
And go from there.
P.S.: please, avoid mirroring this fossil repository to Github and similar. The last thing I want is for people to get some malware because somebody decides to typosquat my code there.