We've managed the mail server in question with Rackspace for over 10 years. We're considering moving to their cloud servers to free up the dedicated hardware it's currently on.
Rackspace admits that the IPs which are given with their cloud servers will very likely be on DNSBLs. As a solution, they've offered free Bronze accounts with SendGrid for each server. Hence my question to this list for their experience with ESPs like SendGrid. Alternatively, Rackspace has offered that we setup a private RackConnect network between a Cloud Server and our existing dedicated firewall with clean public-facing IPs. However the drawback of this scenario is the need to use port mapping for sites which are hosted on the same server, which is only a drawback insofar that we'll need to adapt our systems to work with sites not hosted on port 80 internally. This would also solve the problem of each Cloud Server having a maximum of 5 IP addresses assigned (apparently a Xen limitation?). Quinn
