[exim] exim attachment handling

I have a question regarding Exim (as a relay,) and the handling of
attachments. First though, some background:

Production environment
----------------------

1. Windows 2000 application server using VB code to send emails with
pdf attachments
2. Solaris 9 internal relay running Exim 4.51
3. Windows 2000 upstream relay running an ancient flavour of a
commercial smtp filtering package

Currently, a user of the win2k app server can generate an email with a
pdf attachment. I believe the app code uses the much maligned cdonts
to produce some sort of mimeole encoding (I can't really elaborate
since windows voodoo is a little beyond me.) The email rockets through
the internal Exim relay. Exim sees that this is a non-local delivery
and fires it to the upstream relay. This, in turn, delivers the email
to the final destination (somewhere out on the 'net.) The recipient
receives the email and does clever things with the pdf. All is well -
big smiles all round.

Testing environment
----------------------

1. Windows 2000 application server using VB code to send emails with
pdf attachments (no change)
2. Solaris 9 internal relay running Exim 4.51 (no change)
3. A new upstream relay appliance (destined to replace the ancient win2k relay)

The problem
----------------------

Now...in the testing environment, the same message generation process
fires the email through another internal Exim relay. Initially, this
relay was running 4.51, but I have since compiled and tested against
4.69 too. Unfortunately, the new relay appliance chokes on the email
with a complaint about uuencoded attachments. However, if the app
server sends the email directly via the new relay appliance (i.e.
removing the Exim relay from the equation,) the email arrives at the
final destination normally.

As far as I know, attachments are encoded and decoded by the client.
The mta simply handles the delivery as-is. Obviously, if the mta is
virus/malware scanning, then it has to decode the attachment...but
generally, the mta is passive as far as the attachment is concerned.
Is there some Exim attachment handling that I'm missing? For the life
of me, I can't explain this behaviour (from an Exim perspective.)

The vendor of the new appliance has not been particularly helpful so
far. The finger has been pointed at Exim as the cause of the problem
and their support staff clearly stated that uuencoded attachments are
not supported. I have requested an engineering escalation because I
can't understand firstly, why uuencoding is not supported and
secondly, I can't see why this problem would occur. So far, my
questions have fallen on deaf ears, so I thought it wise to turn to
the list in the meantime.

Any help/suggestions gratefully received!

Thanks

Paul