In years past, many RBLs would categorize IP Addresses by type. e.g. dynamic IPs assigned to DSL/Cable subscribers. This would enable a receving SMTP server to check if the email came from an ISP subscriber, rather than an email server. If so, it was usually a good guage that the email was "spammy", because it was sent by a subscriber's infected computer.
In this sense, it was (and may still be) possible to simply block, or at least score differently, email from any/all AWS Elasic IPs.