I’m running a small MiB install on a DigitalOcean droplet, MiB v0.43 and a fully patched OS. The administrative interface shows not configuration errors, and the server has been operational for about 7 months. For the most part, mail is flowing properly.
One of my users, in an attempt to diagnose another problem (server IP was blacklisted by another host), sent a message from her MiB account to her own MiB account, using a properly configured mail client (not the web interface).
The message failed to deliver (I’ve obscured the sender and domain, but they are correct):
As I read this, the MiB server itself tried to use 184.108.40.206 (based on the MX record for the domain, I assume), but that is not a correct IP, nor is it known to me. As usual with MiB, it is serving as the domain’s DNS server. Admin interface reports:
Nameserver glue records are correct at registrar. [ns1/ns2.mydomain ↦ myIP]
Domain resolves to box’s IP address. [box.mydomain ↦ myIP]
Reverse DNS is set correctly at ISP. [myIP ↦ box.mydomain]
The DANE TLSA record for incoming mail is correct (_25._tcp.box.mydomain).
If I ssh to my server and ask it to resolve box.mydomain or mydomain, the IP address is correct. If I ask it to tell me the MX for the domain, the response is correct:
Name Server: NS1.BOX.MYDOMAIN
Name Server: NS2.BOX.MYDOMAIN
My question: in a case like this – self-send to one’s own account using one’s own account – where in the process would an incorrect IP resolution occur, and where should I begin troubleshooting?