OK, first, I'm confused why you would tell me to turn on debug mode for Dovecot, but ask for output from maillog (especially as that's what I provided in my original post), but here it is:
[01:05:45 root@server log]# tail -f maillog
Oct 19 01:08:30 server postfix/smtpd[13443]: connect from subscriber1.server[1.2.3.4]
Oct 19 01:08:31 server postfix/smtpd[13443]: Anonymous TLS connection established from subscriber1.server[1.2.3.4]: TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)
Oct 19 01:08:31 server postfix/smtpd[13443]: A818FC547F1: client=subscriber1.server[1.2.3.4]
Oct 19 01:08:31 server postfix/cleanup[12598]: A818FC547F1: message-id=<1539911288.2698.34.camel@wrathall>
Oct 19 01:08:31 server postfix/qmgr[24863]: A818FC547F1: from=<subscriber1@external-domain.com>, size=2303, nrcpt=1 (queue active)
Oct 19 01:08:31 server postfix/10025/smtpd[13423]: connect from mlmmj.server[127.0.0.1]
Oct 19 01:08:31 server postfix/10025/smtpd[13423]: F002BC547F3: client=mlmmj.server[127.0.0.1]
Oct 19 01:08:31 server postfix/cleanup[12826]: F002BC547F3: message-id=<1539911288.2698.34.camel@wrathall>
Oct 19 01:08:31 server postfix/10025/smtpd[13423]: disconnect from mlmmj.server[127.0.0.1]
Oct 19 01:08:31 server postfix/qmgr[24863]: F002BC547F3: from=<subscriber1@external-domain.com>, size=2855, nrcpt=1 (queue active)
Oct 19 01:08:31 server amavis[6364]: (06364-17) Passed CLEAN {RelayedInbound}, [1.2.3.4]:58130 [24.86.239.82] <subscriber1@external-domain.com> -> <test20181007@mlmmj-domain.com>, Queue-ID: A818FC547F1, Message-ID: <1539911288.2698.34.camel@wrathall>, mail_id: k1VSeKeJwcJI, Hits: -, size: 2303, queued_as: F002BC547F3, dkim_sd=dkim:mail.external-domain.com, 83 ms
Oct 19 01:08:31 server amavis[6364]: (06364-17) Passed CLEAN, <subscriber1@external-domain.com> -> <test20181007@mlmmj-domain.com>, Hits: -, tag=-100, tag2=3.5, kill=3.5, queued_as: F002BC547F3, L/Y/0/0
Oct 19 01:08:31 server postfix/amavis/smtp[13064]: A818FC547F1: to=<test20181007@mlmmj-domain.com>, relay=127.0.0.1[127.0.0.1]:10024, delay=0.42, delays=0.33/0/0/0.09, dsn=2.0.0, status=sent (250 2.0.0 from MTA(smtp:[127.0.0.1]:10025): 250 2.0.0 Ok: queued as F002BC547F3)
Oct 19 01:08:31 server postfix/qmgr[24863]: A818FC547F1: removed
Oct 19 01:08:32 server postfix/pipe[13468]: F002BC547F3: to=<test20181007@mlmmj-domain.com>, relay=mlmmj, delay=0.02, delays=0/0.01/0/0.01, dsn=2.0.0, status=sent (delivered via mlmmj service)
Oct 19 01:08:32 server postfix/qmgr[24863]: F002BC547F3: removed
Oct 19 01:08:32 server postfix/10025/smtpd[13351]: 0A8B7C547F1: client=mlmmj.server[127.0.0.1]
Oct 19 01:08:32 server postfix/cleanup[12598]: 0A8B7C547F1: message-id=<1539911288.2698.34.camel@wrathall>
Oct 19 01:08:32 server postfix/10025/smtpd[13351]: disconnect from mlmmj.server[127.0.0.1]
Oct 19 01:08:32 server postfix/qmgr[24863]: 0A8B7C547F1: from=<test20181007+bounces-2-subscriber1=external-domain.com@mlmmj-domain.com>, size=4069, nrcpt=1 (queue active)
Oct 19 01:08:32 server amavis[11837]: (11837-06) Passed CLEAN {RelayedInternal}, MLMMJ LOCAL [127.0.0.1] [24.86.239.82] <test20181007+bounces-2-subscriber1=external-domain.com@mlmmj-domain.com> -> <subscriber1@external-domain.com>, Message-ID: <1539911288.2698.34.camel@wrathall>, mail_id: Dk0HH2Sv2IoY, Hits: -, size: 3068, queued_as: 0A8B7C547F1, dkim_new=dkim:mail.iredmail.server, 44 ms
Oct 19 01:08:32 server amavis[11837]: (11837-06) Passed CLEAN, <test20181007+bounces-2-subscriber1=external-domain.com@mlmmj-domain.com> -> <subscriber1@external-domain.com>, Hits: -, tag=0, tag2=0, kill=0, queued_as: 0A8B7C547F1, L/0/0/0
Oct 19 01:08:32 server postfix/smtpd[13443]: disconnect from subscriber1.server[1.2.3.4]
Oct 19 01:08:32 server postfix/10025/smtpd[12964]: connect from mlmmj.server[127.0.0.1]
Oct 19 01:08:32 server postfix/10025/smtpd[12964]: 2019DC547F3: client=mlmmj.server[127.0.0.1]
Oct 19 01:08:32 server postfix/cleanup[12826]: 2019DC547F3: message-id=<1539911288.2698.34.camel@wrathall>
Oct 19 01:08:32 server postfix/qmgr[24863]: 2019DC547F3: from=<test20181007+bounces-2-subscriber2=shaw.ca@mlmmj-domain.com>, size=4065, nrcpt=1 (queue active)
Oct 19 01:08:32 server amavis[11837]: (11837-06-2) Passed CLEAN {RelayedInternal}, MLMMJ LOCAL [127.0.0.1] [24.86.239.82] <test20181007+bounces-2-subscriber2=shaw.ca@mlmmj-domain.com> -> <subscriber2@shaw.ca>, Message-ID: <1539911288.2698.34.camel@wrathall>, mail_id: mysNGXnY0qMh, Hits: -, size: 3068, queued_as: 2019DC547F3, dkim_new=dkim:mail.iredmail.server, 84 ms
Oct 19 01:08:32 server amavis[11837]: (11837-06-2) Passed CLEAN, <test20181007+bounces-2-subscriber2=shaw.ca@mlmmj-domain.com> -> <subscriber2@shaw.ca>, Hits: -, tag=0, tag2=0, kill=0, queued_as: 2019DC547F3, L/0/0/0
Oct 19 01:08:32 server postfix/smtpd[13445]: disconnect from unknown[196.201.19.50]
Oct 19 01:08:32 server postfix/10025/smtpd[12964]: 34928C547F6: client=mlmmj.server[127.0.0.1]
Oct 19 01:08:32 server postfix/cleanup[12598]: 34928C547F6: message-id=<1539911288.2698.34.camel@wrathall>
Oct 19 01:08:32 server postfix/qmgr[24863]: 34928C547F6: from=<test20181007+bounces-2-subscriber3=gmail.com@mlmmj-domain.com>, size=4069, nrcpt=1 (queue active)
Oct 19 01:08:32 server amavis[11837]: (11837-06-3) Passed CLEAN {RelayedInternal}, MLMMJ LOCAL [127.0.0.1] [24.86.239.82] <test20181007+bounces-2-subscriber3=gmail.com@mlmmj-domain.com> -> <subscriber3@gmail.com>, Message-ID: <1539911288.2698.34.camel@wrathall>, mail_id: rQwZiunY1OVn, Hits: -, size: 3068, queued_as: 34928C547F6, dkim_new=dkim:mail.iredmail.server, 80 ms
Oct 19 01:08:32 server amavis[11837]: (11837-06-3) Passed CLEAN, <test20181007+bounces-2-subscriber3=gmail.com@mlmmj-domain.com> -> <subscriber3@gmail.com>, Hits: -, tag=0, tag2=0, kill=0, queued_as: 34928C547F6, L/0/0/0
Oct 19 01:08:32 server postfix/10025/smtpd[12964]: 492A4C547F7: client=mlmmj.server[127.0.0.1]
Oct 19 01:08:32 server postfix/cleanup[12826]: 492A4C547F7: message-id=<1539911288.2698.34.camel@wrathall>
Oct 19 01:08:32 server postfix/qmgr[24863]: 492A4C547F7: from=<test20181007+bounces-2-susan=example.com@mlmmj-domain.com>, size=4079, nrcpt=1 (queue active)
Oct 19 01:08:32 server amavis[11837]: (11837-06-4) Passed CLEAN {RelayedInternal}, MLMMJ LOCAL [127.0.0.1] [24.86.239.82] <test20181007+bounces-2-susan=example.com@mlmmj-domain.com> -> <susan@example.com>, Message-ID: <1539911288.2698.34.camel@wrathall>, mail_id: rhcmHtxkqev8, Hits: -, size: 3068, queued_as: 492A4C547F7, dkim_new=dkim:mail.iredmail.server, 80 ms
Oct 19 01:08:32 server amavis[11837]: (11837-06-4) Passed CLEAN, <test20181007+bounces-2-susan=example.com@mlmmj-domain.com> -> <susan@example.com>, Hits: -, tag=0, tag2=0, kill=0, queued_as: 492A4C547F7, L/0/0/0
Oct 19 01:08:32 server postfix/pipe[13335]: 492A4C547F7: to=<susan@example.com>, relay=dovecot, delay=0.03, delays=0/0/0/0.02, dsn=5.1.1, status=bounced (user unknown)
Oct 19 01:08:32 server postfix/cleanup[12598]: 4F26BC547F9: message-id=<20181019010832.4F26BC547F9@mlmmj.server>
Oct 19 01:08:32 server postfix/bounce[13481]: 492A4C547F7: sender non-delivery notification: 4F26BC547F9
Oct 19 01:08:32 server postfix/qmgr[24863]: 4F26BC547F9: from=<>, size=6048, nrcpt=1 (queue active)
Oct 19 01:08:32 server postfix/qmgr[24863]: 492A4C547F7: removed
Oct 19 01:08:32 server postfix/pipe[13468]: 4F26BC547F9: to=<test20181007+bounces-2-susan=example.com@mlmmj-domain.com>, relay=mlmmj, delay=0.01, delays=0/0/0/0.01, dsn=2.0.0, status=sent (delivered via mlmmj service)
Oct 19 01:08:32 server postfix/qmgr[24863]: 4F26BC547F9: removed
Oct 19 01:08:32 server postfix/10025/smtpd[12964]: 5DC87C547F7: client=mlmmj.server[127.0.0.1]
Oct 19 01:08:32 server postfix/cleanup[12826]: 5DC87C547F7: message-id=<1539911288.2698.34.camel@wrathall>
Oct 19 01:08:32 server postfix/qmgr[24863]: 5DC87C547F7: from=<test20181007+bounces-2-craig=example.net@mlmmj-domain.com>, size=4061, nrcpt=1 (queue active)
Oct 19 01:08:32 server amavis[11837]: (11837-06-5) Passed CLEAN {RelayedInternal}, MLMMJ LOCAL [127.0.0.1] [24.86.239.82] <test20181007+bounces-2-craig=example.net@mlmmj-domain.com> -> <craig@example.net>, Message-ID: <1539911288.2698.34.camel@wrathall>, mail_id: TydeR49PvNwe, Hits: -, size: 3068, queued_as: 5DC87C547F7, dkim_new=dkim:mail.iredmail.server, 82 ms
Oct 19 01:08:32 server amavis[11837]: (11837-06-5) Passed CLEAN, <test20181007+bounces-2-craig=example.net@mlmmj-domain.com> -> <craig@example.net>, Hits: -, tag=0, tag2=0, kill=0, queued_as: 5DC87C547F7, L/0/0/0
Oct 19 01:08:32 server postfix/pipe[13335]: 5DC87C547F7: to=<craig@example.net>, relay=dovecot, delay=0.02, delays=0/0/0/0.01, dsn=5.1.1, status=bounced (user unknown)
Oct 19 01:08:32 server postfix/cleanup[12598]: 62438C547F9: message-id=<20181019010832.62438C547F9@mlmmj.server>
Oct 19 01:08:32 server postfix/bounce[13481]: 5DC87C547F7: sender non-delivery notification: 62438C547F9
Oct 19 01:08:32 server postfix/qmgr[24863]: 62438C547F9: from=<>, size=5976, nrcpt=1 (queue active)
Oct 19 01:08:32 server postfix/qmgr[24863]: 5DC87C547F7: removed
Oct 19 01:08:32 server postfix/pipe[13468]: 62438C547F9: to=<test20181007+bounces-2-craig=example.net@mlmmj-domain.com>, relay=mlmmj, delay=0.01, delays=0/0/0/0.01, dsn=2.0.0, status=sent (delivered via mlmmj service)
Oct 19 01:08:32 server postfix/qmgr[24863]: 62438C547F9: removed
Oct 19 01:08:34 server postfix/smtp[13352]: Untrusted TLS connection established to smtp.glb.shawcable.net[64.59.136.136]:25: TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)
Oct 19 01:08:34 server postfix/submission/smtpd[13496]: connect from unknown[102.149.192.55]
Oct 19 01:08:35 server postfix/smtp[13352]: 2019DC547F3: to=<subscriber2@shaw.ca>, relay=smtp.glb.shawcable.net[64.59.136.136]:25, delay=3.5, delays=0/0/2.6/0.94, dsn=2.0.0, status=sent (250 2.0.0 DJHEgPnSQ85hUDJHGgYF6n mail accepted for delivery)
Oct 19 01:08:35 server postfix/qmgr[24863]: 2019DC547F3: removed
^C
[01:08:52 root@server log]#
One more time, susan@example.com and craig@example.net **DO** exist and send and receive email multiple times a day. There are **NO** spelling mistakes.