Let's say SendGrid did get blocked, despite their work to make sure they don't (they have more of vested interest than Amazon do).
If it was costing them serious business, presumably they would change their IPs, etc. Where as Amazon are unlikely to because sending email isn't their core business.
Hard to say. Amazon takes all their AWS products pretty seriously. I am sure they have thought through email deliverability. I wouldn't discount their solution just because their EC2 IPs have been blocked in the past for mass email.
I'm aware of AWS firewall blocks at various sites.
Have a look around at some of the forums for web masters, and you'll find wholesale blacklisting of these server farms is regularly discussed. And implemented.
The presumption being that outbound connects from these server farms are usually something where the VPS has vastly more bandwidth, or bugs, or breaches; where the attackers and the spammers and the scrapers have quickly adopted or targeted these server farms just because of their capabilities.
And yes, there are well-intentioned folks using AWS that are getting painted with this same brush.
They could, but I assume SendGrid has more checks in place to prevent their users from sending unwanted emails. That way SendGrid has little risk of getting blacklisted, whereas a presumably uncontrolled service like Amazon's would.
