[exim] Error relaying large emails over two exim hosts

Environment (all servers running Debian etch w exim4 and latest patches applied).
Both Linux-Servers are running FWBuilder iptable firewalls.

Scenario sending a mail from the WXP PC:
* WindowsXP-PC w Thunderbird (eMail source)
-> MailServer1 (gateway, smart router, connected via DSL Router)
-> MailsServer2 (relay, outgoing eMail) sending emails to the final destination.
Anything works fine until large eMails (or eMail w large attachments / approx > 1MB) are sent.
Whenever large email are sent, the connection once is broken and the sending host (Server1) retries sending the emails on next
queue-run.
From the exim log of Server1:
"R=smart_route T=remote_smtp defer (-46): SMTP error from remote mail server after end of data: host xx.xx.xx.xx [xx.xx.xx.xx]: 421
adomain.eu SMTP incoming data timeout - closing connection"

I did some investigation a few weeks ago and found out that after a while fragmented packets are sent over the Internet line. I
tried some "optimized" MTU settings but could not solve the problem. I do not think it's a problem with the MTU.

Regardless of the problems sending large mail from Server1 to the internet, receiving large eMails is not a problem.

Any help appreciated.