Hi Holger, perhaps you can teach me something about networking. I need
to learn more about it.

Here's a review of some things you have said:

* This host is connected via an SHDSL-Modem (Bridge) to our network.
* i get a "timeout after DATA" error.
* The sender is using an Exchange 2003 Server, but with an
Borderware MXtreme Mailfirewall
* I changed the MTU of my server a bit lower (1448)
* on all our routers and our firewall applied "disable ip path mtu discovery
echo 1 >/proc/sys/net/ipv4/ip_no_pmtu_disc


Ok, first of all, you disabled MTU discovery. The purpose of MTU discovery as I
understand it, is to figure out the largest packet size that can be used for
both endpoints, and for every router and device in between. This value gets set
as "PMTU" (Path MTU). Then there is the MTU setting itself, which I understand to
be the overall maximum packet size that can be used. Therefore PMTU (if used) should
be equal to or less than MTU, correct? But you are not using it.

As I understand it, the MTU used by SHSDL modems is 1500. You sent your MTU to 1448.
Without using path mtu discovery, couldn't this cause problems with the modem using 1500
and your sever using 1448? Or does the other end detect that your server is 1448, and
reset it's PMTU ?

--
Greg