It depends on how exactly you've configured it all to work together. My set up is similar to that as shown at workaround.org, here's what a message looks like in mail.log as it arrives from "out there" and is processed by postfix and dovecot using lmtp to pass it off. You can see the log entries for the lmtp pass off.
Jul 6 04:24:40 node3 postfix/smtpd[22238]: connect from sendinghostname[sendingip]
Jul 6 04:24:40 node3 postfix/smtpd[22238]: Anonymous TLS connection established from sendinghostname[sendingip]: TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)
Jul 6 04:24:40 node3 postfix/smtpd[22238]: DF2FE1EF3C: client=sendinghostname[sendingip]
Jul 6 04:24:40 node3 postfix/cleanup[22241]: DF2FE1EF3C: message-id=<e0f21d60-ea5d-4563-53d5-6e9450540655@sendingdomainname>
Jul 6 04:24:40 node3 spamd[8704]: spamd: connection from localhost [127.0.0.1]:44362 to port 783, fd 5
Jul 6 04:24:40 node3 spamd[8704]: spamd: processing message <e0f21d60-ea5d-4563-53d5-6e9450540655@sendingdomainname> for username:5000
Jul 6 04:24:41 node3 spamd[8704]: spamd: clean message (-0.0/5.0) for username:5000 in 0.3 seconds, 1576 bytes.
Jul 6 04:24:41 node3 spamd[8704]: spamd: result: . 0 - SPF_PASS scantime=0.3,size=1576,user=username,uid=5000,required_score=5.0,rhost=localhost,raddr=127.0.0.1,rport=44362,mid=<e0f21d60-ea5d-4563-53d5-6e9450540655@sendingdomainname>,autolearn=ham autolearn_force=no
Jul 6 04:24:41 node3 postfix/qmgr[3741]: DF2FE1EF3C: from=<username@sendingdomainname>, size=1789, nrcpt=1 (queue active)
Jul 6 04:24:41 node3 postfix/smtpd[22238]: disconnect from sendinghostname[sendingip] ehlo=2 starttls=1 mail=1 rcpt=1 data=1 quit=1 commands=7
Jul 6 04:24:41 node3 dovecot: lmtp(22244): Connect from local
Jul 6 04:24:41 node3 dovecot: lmtp(username@recipientdomain): qpVLDom7XVnkVgAAD1k5Pg: sieve: msgid=<e0f21d60-ea5d-4563-53d5-6e9450540655@sendingdomainname>: stored mail into mailbox 'INBOX'
Jul 6 04:24:41 node3 dovecot: lmtp(22244): Disconnect from local: Successful quit
Jul 6 04:24:41 node3 postfix/lmtp[22243]: DF2FE1EF3C: to=<username@recipientdomain>, relay=receivingservername[private/dovecot-lmtp], delay=0.35, delays=0.3/0.02/0.01/0.01, dsn=2.0.0, status=sent (250 2.0.0 <username@recipientdomain> qpVLDom7XVnkVgAAD1k5Pg Saved)
Jul 6 04:24:41 node3 postfix/qmgr[3741]: DF2FE1EF3C: removed
Jul 6 04:24:41 node3 spamd[8703]: prefork: child states: II