https://bugs.exim.org/show_bug.cgi?id=1696

            Bug ID: 1696
           Summary: verify_sender fails when sender's server is IPv6 only
                    without MX record (delivered via IPv4)
           Product: Exim
           Version: 4.86
          Hardware: x86
                OS: Linux
            Status: NEW
          Severity: bug
          Priority: medium
         Component: Routing
          Assignee: [email protected]
          Reporter: [email protected]
                CC: [email protected]

In a situation where a mail from server A was delivered via a (forwarding)
host, B, to Exim 4.86 running on host C in a line like this:

A  --IPv6-->  B  --IPv4-->  C

and where A has IPv6 connectivity only, sender verification on Exim on host C
fails ("sender verify fail for <nagios@A>: Unrouteable address").

A direct delivery from A to C (which has both IPv4 and IPv6 connectivity) was
not tested and *might* have the same issue.

Server A hat no MX entry in DNS when that happened and only had an AAAA entry.

Arguably, sender verification should still succeed, I think. RFC5321 says "If
an empty list of MXs is returned, the address is treated as if it was
associated with an implicit MX RR, with a preference of 0, pointing to that
host." but makes no assumption on whether that host should be reachable by
either IPv4 or IPv6. So arguably, both should be tested.

Thanks!

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