[c-nsp] tcp retransmissions

[Kern, Tom]

I'm running an Exchange 2k bridgehead server which is journalling all my org's mail(about 100,000 emails a day) through a GRE tunnel between 2 2600 cisco routers over T1 to my ISP and thru the internet.
 
the other side of the connection is saying they only get about 4-5 journaled emails a day.
when i run ethereal on the exchange bridgehead, i see a ton of tcp retransmissions to the journal server.
 
is this due to the GRE tunnel and if so, what can i do about it?
change the MTU orMSS,etc?
 
Thanks alot