My Exim setup runs smoothly for every external domain I deal with, yet my two in-house domains refuse to talk to one another. Each domain sends and receives mail flawlessly everywhere else, but the moment an address on domain-A tries domain-B (or vice-versa) Exim throws a “550 – user not found” even though the mailbox exists and works. Here is what I know so far: • Server: Exim (single host handling both domains) • MX, SPF, DKIM, and DNS records for both domains propagate correctly and pass external tests. • Local delivery to each domain is fine; cross-domain delivery is the only failure. • Logs confirm the 550 rejection at RCPT stage; user databases are intact. • I’ve reviewed the main and virtual-alias sections of the config and nothing obvious jumps out. What I need from you is a clean, sustainable fix. That means: 1. Identify the exact mis-routing, lookup, or aliasing rule that’s blocking internal domain-to-domain mail. 2. Adjust Exim configuration (routers, transports, virtual_user, etc.) so both domains deliver locally without error and without disrupting outside traffic. 3. Provide a concise summary of the change so I can replicate it on staging and future hosts. You will get full SSH access to a test snapshot; once it works there we’ll roll the change into production together (I’ll handle the cut-over, you supervise). Familiarity with split-domain routing, virtual domains, and Exim ACLs will make this a quick win.