Exim - broken pipe. Whose side is the problem on?

Question

Exim

Good afternoon!
There was a problem sending emails to one domain. Exim log says:

H=mxs.h*z.ru [1.2.3.4]: mxs.h*z.ru [1.2.3.4]: Broken pipe

Trying to resend email:

exim -q -v
LOG: MAIN
  Warning: purging the environment.
 Suggested action: use keep_environment.
LOG: queue_run MAIN
  Start queue run: pid=99934
delivering 1c7gl7-000AGI-Np (queue run pid 99934)
Connecting to mxs.h*z.ru [1.2.3.4]:25 ... connected
  SMTP<< 220 **********************************************************************************
  SMTP>> EHLO mail.b-a.ru
  SMTP<< 250-mxs.h*z.ru Hello [2.3.4.5]
         250-SIZE
         250-PIPELINING
         250-DSN
         250-ENHANCEDSTATUSCODES
         250-XXXXXXXA
         250-AUTH NTLM LOGIN
         250-8BITMIME
         250-BINARYMIME
         250 XXXXXXXB
  SMTP>> MAIL FROM:<[email protected]> SIZE=403400
  SMTP>> RCPT TO:<[email protected]*z.ru>
  SMTP>> RCPT TO:<[email protected]*z.ru>
  SMTP>> RCPT TO:<[email protected]*z.ru>
  SMTP>> RCPT TO:<[email protected]*z.ru>
  SMTP>> DATA
  SMTP<< 250 2.1.0 Sender OK
  SMTP<< 250 2.1.5 Recipient OK
  SMTP<< 250 2.1.5 Recipient OK
  SMTP<< 250 2.1.5 Recipient OK
  SMTP<< 250 2.1.5 Recipient OK
  SMTP<< 354 Start mail input; end with <CRLF>.<CRLF>
  SMTP>> writing message and terminating "."
LOG: MAIN
  H=mxs.h*z.ru [1.2.3.4]: mxs.h*z.ru [1.2.3.4]: Broken pipe

Answer 1

The problem was found in Cisco ASA, namely in ESMTP inspection. Disabling the feature restored mail flow. The specific reason remains unclear.

Answer 2

Broken pipe is a broken connection. It can cut the firewall or an error in the firmware of the router (this was observed on Cisco). There may be a problem with the MTU size. But, most likely, the receiving server just does a drop.