When I tried I didn't manage to resolve tnl-online.net at all. The reason for that could have been that the two ns were not in synch - impossible to tell today, since they resolve now, however not valid. f.ex. delphi.tnl-online.net is cname for delphi.tnl-online.com - an mx should point to a record that has an A record - not all resolvers proceed beyond an inital cname since it's not part of the mx-rfc.

Mercury can't see the mx on a domain of mine I added recently. I checked with nslookup on that machine running Mercury and it sees the mx records. see below
It's an older version of Mercury. I redid the domain zone but that didn't help.  Mercury ignores the MX and tries to connect to the primary server.

20:01:28: processing job MO019935
Resolved CN for 'sideaa.com' to 204.52.210.72
Connecting to 204.52.210.72
Connection error.

20:02:16: processing job MO019464
Resolved CN for 'sideaa.com' to 204.52.210.72
Connecting to 204.52.210.72
Connection error.

 
> set type=MX
> sideaa.com


sideaa.com      MX preference = 30, mail exchanger = delphi.tnl-online.net
sideaa.com      MX preference = 10, mail exchanger = titan.tnl-online.net

 
>nslookup on the mercury machine sees it.  I rebooted but that didn't help. 
Why the CN and not MX?

<!---->

The domain tnl-online.net doesn't resolve

sure it does.  my other domains don't have this issue.

C:\>nslookup
Default Server:  catbert.nnmt.net
Address:  65.218.223.4

> set type=all
> tnl-online.net
Server:  catbert.nnmt.net
Address:  65.218.223.4

Non-authoritative answer:
tnl-online.net  text =

        "v=spf1 a:sdictnl.tnl-online.com a:proteus.sdic.org ip4:204.52.210.11 ip4:204.52.210.39  -all"
tnl-online.net  MX preference = 10, mail exchanger = delphi.tnl-online.com
tnl-online.net  MX preference = 30, mail exchanger = titan.tnl-online.com
tnl-online.net
        primary name server = zeus.tnl-online.com
        responsible mail addr = noc02.tnl-online.net
        serial  = 2007091001
        refresh = 3600 (1 hour)
        retry   = 600 (10 mins)
        expire  = 86400 (1 day)
        default TTL = 3600 (1 hour)
tnl-online.net  internet address = 204.52.210.35
tnl-online.net  nameserver = zeus.tnl-online.com
tnl-online.net  nameserver = ns3.tnl-online.com

tnl-online.net  nameserver = ns3.tnl-online.com
tnl-online.net  nameserver = zeus.tnl-online.com
ns3.tnl-online.com      internet address = 65.218.223.109
zeus.tnl-online.com     internet address = 204.52.210.13
>

right.  tnl-online.com should have been the MX.  that was the error. 

thanks.  it's the two machine names titan and delphi that don't resolve.  whew.