Pyramid / MacTCP problems?

[Geoff Collin]

We have just installed MacTCP and can happily talk to all our IP
hosts except our Pyramids ( running a range of OS's between 4.4 and 5 ).
UDP transactions and some TCP connections (telnet) are OK but other TCP
connections fail eg SMTP. It appears to be some form of buffer size 
negotiation failure that occurs during the connection. 
Does anyone have a fix please?

Geoff