Found this log in my internal SMTP log that I suspected as notification message session:
Jun 18 11:07:51 centos1 postfix/smtpd[16340]: connect from sbmf.local.com[192.168.40.33]
Jun 18 11:07:51 centos1 postfix/smtpd[16342]: connect from sbmf.local.com[192.168.40.33]
Jun 18 11:07:51 centos1 postfix/smtpd[16343]: connect from sbmf.local.com[192.168.40.33]
Jun 18 11:07:51 centos1 postfix/smtpd[16344]: connect from sbmf.local.com[192.168.40.33]
Jun 18 11:07:51 centos1 postfix/smtpd[16345]: connect from sbmf.local.com[192.168.40.33]
Jun 18 11:12:53 centos1 postfix/smtpd[16340]: timeout after EHLO from sbmf.local.com[192.168.40.33]
Jun 18 11:12:53 centos1 postfix/smtpd[16340]: disconnect from sbmf.local.com[192.168.40.33]
Jun 18 11:12:53 centos1 postfix/smtpd[16342]: timeout after EHLO from sbmf.local.com[192.168.40.33]
Jun 18 11:12:53 centos1 postfix/smtpd[16342]: disconnect from sbmf.local.com[192.168.40.33]
Jun 18 11:12:53 centos1 postfix/smtpd[16343]: timeout after EHLO from sbmf.local.com[192.168.40.33]
Jun 18 11:12:53 centos1 postfix/smtpd[16343]: disconnect from sbmf.local.com[192.168.40.33]
Jun 18 11:12:53 centos1 postfix/smtpd[16344]: timeout after EHLO from sbmf.local.com[192.168.40.33]
Jun 18 11:12:53 centos1 postfix/smtpd[16344]: disconnect from sbmf.local.com[192.168.40.33]
Jun 18 11:12:53 centos1 postfix/smtpd[16345]: timeout after EHLO from sbmf.local.com[192.168.40.33]
Jun 18 11:12:53 centos1 postfix/smtpd[16345]: disconnect from sbmf.local.com[192.168.40.33]
Note:
sbmf.local.com[192.168.40.33] is server with Brightmail installed.
centos1 is internal SMTP server
Why the SMTP session timed out after HELO? FYI normal mail (both incoming and outgoing) can be delivered successfully.