When a new email comes in, it is processed by your MX server (eg mx1, mxa) and then handed over to one of our outbound relay servers to deliver the email.
The IPs of our outbound relay servers are listed in dnswl.org, a popular whitelist used by most email providers to ensure that emails are delivered.
Our current sending IPs can be found in the SPF record _spf.emailforwardmx.com, however these are subject to change at any time.
The vast majority of the emails that we forward are delivered by our outbound relay servers, as we have good IP reputation with most major providers such as Google, Microsoft, Yahoo, Fastmail, etc.
However, if our relay server receives a rejection message, it then passes the message off to another provider, Mail Baby, who maintains a pool of high-reputation IPs for email deliverability.
If Mail Baby can't deliver the message using their own IPs, then they will pass it off to MailChannels, another provider who maintains a pool of high-reputation IPs as well.
If Mail Baby and MailChannels can't deliver the message, then the issue is not an IP reputation issue, it's an issue with the content of the message itself.