$ grep B6521360291 /var/log/mail.log 
May 29 09:36:52 $MSERVER postfix/submission/smtpd[352]: B6521360291: client=XXXXXXX.XXX.XXXXXXXX.XXX.XXX[XXX.XXX.XXX.XXX], sasl_method=CRAM-MD5, sasl_username=$USER
May 29 09:36:53 $MSERVER postfix/cleanup[350]: B6521360291: message-id=<20180529093651.2f437686f055a3759bdfac9c@kuzuore.com>
May 29 09:36:53 $MSERVER postfix/qmgr[32708]: B6521360291: from=<$USER@kuzuore.com>, size=808, nrcpt=2 (queue active)
May 29 09:36:55 $MSERVER postfix/smtp[355]: B6521360291: to=<$AUSER@gmail.com>, relay=gmail-smtp-in.l.google.com[74.125.204.26]:25, delay=2.6, delays=0.42/0.03/1.1/1.1, dsn=2.0.0, status=sent (250 2.0.0 OK 1527554215 3-v6si30592000plz.337 - gsmtp)
May 29 09:36:58 $MSERVER postfix/smtp[354]: B6521360291: to=<$BUSER@DOMAIN>, relay=YYYYY.YYY.YYYYYYYYY.YYY[YYY.YYY.YYY.YYY]
:25, delay=6, delays=0.42/0.01/0.69/4.8, dsn=2.0.0, status=sent (250 2.0.0 Ok: queued as 751291D1)
May 29 09:36:58 $MSERVER postfix/qmgr[32708]: B6521360291: removed