If I understand correctly, you need to route all Outgoing email to a specific server instead of using the MX records for delivery. This can be done using SMTP Routes, here is how:
1) Start on the SMTP Routes page of the Network tab
2) Review the current entries:
- Verify all domains associated with Inbound Mail are accounted for
- You can compare this to the Recipient Access Table (RAT) on the Mail Policies tab, which shows all currently configured Inbound domains
2a) If an inbound domain is missing, determine if it should be routed using DNS or to static IPs and then create an SMTP Route using this information
3) Set All Other Domains to the desired destination IP
4) Submit & Commit
Now all Inbound mail will use the domain-specific SMTP Routes, while all outbound mail will use the All Other Domains entry.
I hope this helps!
- Jackie