Small E-Mail SendReceive CorrectlyLarge E-Mail Messages Do Not

Small E-Mail Send/Receive CorrectlyLarge E-Mail Messages Do Not

In this example, our firewall is either operating an MTA or forwarding connections to a DNAT-ed SMTP server. Small e-mail messages can be sent and received correctly by the mail server; large messages repeatedly fail.

This is almost always caused by an MTU setting on the firewall, typically in DSL, or PPPoE environments where the overhead of this environment reduces the maximum unit size you can transmit over your upstream connection. This also can occur with VPN environments, such as PPTP. The fix for this situation is fairly straight forward, and we discuss it in more detail in the VPN chapter. For brevity's sake, here's the fix:

 # where eth0 is the external interface $IPTABLES -A OUTPUT -o eth0 -p tcp \       --tcp-flags SYN,RST SYN -j TCPMSS \       clamp-mss-to-pmtu