Running Sendmail 5.2a-1

My mail sessions to one mail server (a Barracuda firewall) are dying if 
there is a large attachment

I've turned on tcp_read and tcp_write logging.  This is the log at the 
end of the session.  Looks like SM is happily sending MIME-encoded data 
then something stops after 108s

Have I stopped sending, or have they stopped receiving?

( lots of lines )
1 16:01:24.00:Info:-1300952144: tcp_write: [100] 
THMdzR7s7pAT\m\j37Z9xX21prMVVFJyTnIrxn9nDwknhfwZbxxxqhKgsB9BXqsOqMgwgzxWHL7zdjeDSibWoazHbL5U\m\jRy2AKbZa
  1 16:01:24.00:Info:-1300952144: tcp_write: [100] 
rvjywycdzWBLflMyDknocV594x/aq+D3gjVv+Ee1vx/p0V7k7rRLpWkUjqCAa1Sk9uho\m\j63Jq3Y9avtSkkQt2XrisW+v7qfKxRk
  1 16:03:12.00:Info:-1300952144: tcp_check_open got pullhup event on 
socket (Err Code Zero)
  1 16:03:12.00:Info:-1300952144: tcp: Slow write 108 (108) seconds, 
48046/63998 bytes sent
  1 16:03:12.00:Info:-1300952144: Only wrote 48046/63998 bytes in that 
packet 1008802, retry 0, upto 0
  1 16:03:12.00:Info:-1300952144: Only wrote 0/15952 bytes in that 
packet 1008802, upto=48046, in 109 seconds
  1 16:03:12.00:Info:-1300952144: send_body failed (to=216.8.160.21 
TCPWrite failed 0/15952, tot=1008802 upto=48046 109 sec Connection was 
closed. writeopencheck)
  1 16:03:12.00:Info:-1300952144: send: finished sending a message retry 
all (to=216.8.160.21 TCPWrite failed 0/15952, tot=1008802 upto=48046 109 
sec Connection was closed. writeopencheck)
  1 16:03:12.00:Info:-1300952144: send: [177780956] going to set 
msg_later 1 [177780956]
  1 16:03:12.00:Info:-1300952144: msg: log_reason [177780956] Later: 
216.8.135.82 HIDDEN@ul@mnsi.net> <jbodchon@ventraplastics.com> 0  
"to=216.8.160.21 TCPWrite failed 0/15952, tot=1008802 upto=48046 109 sec 
Connection was closed. writeopencheck"
  1 16:03:12.00:Info:-1300952144: q[177780956] send out->QUIT
  1 16:03:12.00:Info:-1300952144: send: not open [177780956]
  1 16:03:12.00:Info:-1300952144: Send_finish called from = 
HIDDEN@ul@mnsi.net>
  1 16:03:12.00:Info:-1300952144: Send_finish, reque = 0
  1 16:03:12.00:Info:-1300952144: msg_fque_todisk qnum=177780956 inmem 
was already false
  1 16:03:12.00:Info:-1300952144: msg: rewriting msg file
  1 16:03:12.00:Info:-1300952144: msg_open 
(/usr/local/surgemail/work/56/q177780956.idx_new)
  1 16:03:12.00:Info:-1300952144: que_write q=177780956 idx_written 1
  1 16:03:12.00:Info:-1300952144: msg_reque last qnum=177780956 busy=0
  1 16:03:12.00:Info:-1300952144: send_thread, send_finish
  1 16:03:12.00:Info:-1300952144: mlink_slow: Took 109 seconds
  1 16:03:12.00:Info:-1300952144: online_mlink_tell_remove

Who threw the pullhup event?  My Surgemail server or the TCP session on 
the other end?