Winlink 2000 (WL2K) using what is called B2F compressed binary forwarding. This is an extension of protocol developed by F6FBB and incorporated into
his PBBS software. Most digital systems have adopted FBB type forwarding.
The Winlink Classic MBO software that NTSD uses only supports up to B1F binary compressed forwarding.  The net result of trying to down message
from a NTSD MBO using B2F forwarding is a lot of binary compression errors resulting in a totally garbled and unreadable message.
Airmail can set up various levels of forwarding.  The 3.4.062 version for instance will let you set B1F forwarding to access a NTSD MBO but that version
of the software automatically defaults back to B2F when you shut Airmail down and restart it.
Are you using the attached SystemNTSDPA Airmail initialization file to help you select the frequencies and "dial" frequencies for the MBO you are using?  
If you are using this, Airmail will have two systems, one called HAM which is used for accessing a WL2K HF Pactor Gateway (or VHF RMS Packet gateway)
and one called NTSDPA which should be used for NTSD.  NTSSPA stands for NTS Digital Pacific Area and it has all the different Pacific area MBO/hubs and
their frequencies.  This allows Airmail to display the correct "dial" frequency and mode (LSB/USB) for the MBO/hub you are connecting to.  And if you also
have your radio set up for computer control by the Airmail software it will automatically put your radio on the right "dial" frequency and mode.    It is then
possible using the 3.3.081 version of Airmail to independently set   B1F for the NTSDPA system and B2F for the HAM (WL2K) system and this version of
Airmail will keep those settings without defaulting back to B2F.
So if you are not using this file, copy and paste it into the  C:\Program Files\Airmail  directory that Airmail creates upon installation.  The next time you
start up Airmail, a small window will pop up asking for your NTSDPA callsign which of course is your amateur callsign.   >From there in the "tools"  menu,
then "options" and then "settings"  under "System Settings" you will see two system tabs, again one called HAM and one called NTSDPA.  You can then
select the appropriate forwarding protocol for each system (B2F for HAM and B1F for NTSDPA) click on "apply" followed by "OK" and you are all set.
The fact that changes like this to the functionality of the Airmail software and its friendliness to NTSD are the exact reasons why NTSD maintains an
archive of current and past versions of Airmail.  We want to be able to continue using it even if future versions get modified to become less NTSD friendly. 
For a similar reason we also have archived the Winlink Classic software and various Windows operating systems starting with Windows 95 up through
Win 98, Win 2000 and Windows XP.
Dave  WB2FTX
March 2013