Scale isn't the main problem, interoperability is. If you've solved interoperability (i.e. you've got SPF, DKIM, etc working so gmail.com and outlook.com will receive emails sent from your system) then you're in good shape.
Not that running large systems is necessarily easy, but it doesn't have enemies the way interoperability has enemies. Scale is a merely conventional problem that Google and Microsoft aren't making worse for Linux users. Nobody's pushing back, trying to make you fail; your only foe is savage reality.
And man-vs-savage-reality is a pretty nice conflict to be involved in, compared to man-vs-man.