Notes/Domino 6 and 7 Forum : SMTP Receive failed (did not timeout)

Since upgrade to R6 we have this problem. No messages received from one particular external mail server, from all other servers mail is received OK. They are using MS Outlook and MS Exchange Server V5.5 SP4 on NT and external gateway is PMDF V6.2. On our end there is a Sidewinder firewall which is showing normal communication with this server. Their users are receiving "Error reading SMTP packet; response to dot-stuffed message expected". My log follows:

02/24/2003 09:47:48 AM SMTP Server [0202:0008-0204] Processing in Connected state
02/24/2003 09:47:48 AM SMTP Server [0202:0008-0204] EHLO command received
02/24/2003 09:47:48.92 AM [0202:0008-011E] SMTP CITask StateMachine> Received 49 bytes from 123.456.1.100
[0202:0008-011E] R: MAIL FROM:<anyone@abc.com> SIZE=3072
02/24/2003 09:47:48.93 AM [0202:0008-011E] SMTP CITask StateMachine> Received 38 bytes from 123.456.1.100
[0202:0008-011E] R: RCPT TO:<me@xyz.com>
02/24/2003 09:47:48.93 AM [0202:0008-011E] SMTP CITask StateMachine> Received 38 bytes from 123.456.1.100
> 02/24/2003 09:47:48.95 AM [0202:0008-011E] SMTP CITask StateMachine> Received 4 bytes from 123.456.1.100
[0202:0008-011E] R: DATA
02/24/2003 09:47:48.96 AM [0202:0008-0204] SMTP CITask StateMachine> Sent 297 bytes to 123.456.1.100
[0202:0008-0204] S: 250 anyone@abc.com... Sender OK<CRLF>
[0202:0008-0204] S: 250 me@xyz.com... Recipient OK<CRLF>
[0202:0008-0204] S: 354 Enter message, end with "." on a line by itself<CRLF>
02/24/2003 09:47:48.99 AM [0202:0008-0204] SMTP CITask RecvErrorHandler> ERROR: (123.456.1.100) Receive failed (did not timeout), error = 0A02h (Remote system no longer responding)
02/24/2003 09:47:48.99 AM [0202:0008-0204] SMTP CITask RecvErrorHandler> Running default hanlder, error = 0A02h (Remote system no longer responding)

No one seems able to figure this one out.
TIA for any ideas.
Pat