It's more likely that we're approaching it from different angles :)
> There are no MX records, just a simple forward rule for any mail with my
> domain name to be forwarded to my Gmail address
Something must answer on the SMTP level for that to occur. So that would make three systems:
Sender -> Forwarder -> Recipient
That Forwarder doesn't do anything but act on a rule is irrelevant.
One of the servers receiving the message, Forwarder or Recipient, is performing recipient validation. That is, it is checking the to address on the mail and rejecting it if it can't find a match.
That takes us out of the DNS realm entirely unless the server handling the message is incorrectly set.
We should be able to determine which it is from the NDR. If the server quoted on the NDR belongs to Sender then the Forwarder is at fault. If it belongs to Forwarder then the Recipient is at fault.
If an MX record isn't defined the A record bound to the domain name itself will be used. e.g.
If this returns nothing:
nslookup -q=mx domain.com
This will be used:
nslookup -q=a domain.com
That doesn't leave us in a very good place. If you don't administer any of the systems then you simply cannot check and fix this. Calls would have to be raised with those that do administer the systems to do that.
Chris
