Sendmail outputs "collect: premature EOM: unexpected close"

Issue

• A smtp client does not transmit some mails. I look receiver host's /var/log/maillog, and the following messages are output.

  • Pattern 1

  May 24 11:41:36 hostname sendmail[2706]: o4O2aLvc002706: collect: premature EOM: unexpected close
  May 24 11:41:36 hostname sendmail[2706]: o4O2aLvc002706: collect: unexpected close on connection from [10.xx.xxx.x], sender=<*********@**.****.co.jp>
  

  • Pattern 2

  Feb 26 08:32:53 hostname sendmail[18713]: o1PNNejK018713: collect: premature EOM: Connection reset by [10.xx.xxx.xxx]
  Feb 26 08:32:53 hostname sendmail[18713]: o1PNNejK018713: SYSERR(root): collect: I/O error on connection from [10.xx.xxx.xxx], from=<xxxx@xxx.xxx.xxx.com>