Hi Greg,

i´m sorry. i´m no networking geek... So i don´t think i could
answer your questions. All my informations are from group-posts... :-)

btw: my problem still does exist. some network geeks told me, that it
must be a firewall problem. and no postfix configuration error.

Holger

Greg Hackney schrieb:

> 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