I followed your advice and was able to get mail to send out from @thatoneguy.com to external domains, however when trying to send mail within the domain, postfix throws an unknown user error and bounces the mail back. I've added thatoneguy.com to /etc/hosts hoping that might help find the user, but still no luck.
journalctl -u postfix
-- Logs begin at Wed 2019-04-17 13:30:37 EDT, end at Wed 2019-04-17 13:50:06 EDT. --
Apr 17 13:31:12 mx systemd[1]: Starting Postfix Mail Transport Agent...
Apr 17 13:31:26 mx postfix/postfix-script[9670]: warning: /var/spool/postfix/etc/hosts and /etc/hosts differ
Apr 17 13:31:27 mx postfix/postfix-script[9684]: starting the Postfix mail system
Apr 17 13:31:27 mx postfix/master[9686]: daemon started -- version 2.10.1, configuration /etc/postfix
Apr 17 13:31:27 mx systemd[1]: Started Postfix Mail Transport Agent.
Apr 17 13:49:54 mx postfix/postscreen[11178]: CONNECT from [127.0.0.1]:50850 to [127.0.0.1]:25
Apr 17 13:49:54 mx postfix/postscreen[11178]: WHITELISTED [127.0.0.1]:50850
Apr 17 13:49:54 mx postfix/smtpd[11179]: connect from mx.thatoneguy.com[127.0.0.1]
Apr 17 13:49:54 mx postfix/smtpd[11179]: 44kqWQ5FKJz4xHTp: client=mx.thatoneguy.com[127.0.0.1]
Apr 17 13:49:54 mx postfix/cleanup[11183]: 44kqWQ5FKJz4xHTp: message-id=<25e6-5cb76780-1-7087f800@224628920>
Apr 17 13:49:54 mx postfix/smtpd[11179]: disconnect from mx.thatoneguy.com[127.0.0.1]
Apr 17 13:49:54 mx postfix/qmgr[9688]: 44kqWQ5FKJz4xHTp: from=<klayne@thatoneguy.com>, size=1000, nrcpt=1 (queue active)
Apr 17 13:49:55 mx postfix/10025/smtpd[11196]: connect from mx.thatoneguy.com[127.0.0.1]
Apr 17 13:49:55 mx postfix/10025/smtpd[11196]: 44kqWR3dtQz4xHV2: client=mx.thatoneguy.com[127.0.0.1]
Apr 17 13:49:55 mx postfix/cleanup[11183]: 44kqWR3dtQz4xHV2: message-id=<25e6-5cb76780-1-7087f800@224628920>
Apr 17 13:49:55 mx postfix/qmgr[9688]: 44kqWR3dtQz4xHV2: from=<klayne@thatoneguy.com>, size=2142, nrcpt=1 (queue active)
Apr 17 13:49:55 mx postfix/10025/smtpd[11196]: disconnect from mx.thatoneguy.com[127.0.0.1]
Apr 17 13:49:55 mx postfix/amavis/smtp[11190]: 44kqWQ5FKJz4xHTp: to=<test@thatoneguy.com>, relay=127.0.0.1[127.0.0.1]:10024, delay=0.97, delays=0.18/0.04/0.01/0.74, 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 44kqWR3dtQz4xHV2)
Apr 17 13:49:55 mx postfix/qmgr[9688]: 44kqWQ5FKJz4xHTp: removed
Apr 17 13:50:00 mx postfix/pipe[11197]: 44kqWR3dtQz4xHV2: to=<test@thatoneguy.com>, relay=dovecot, delay=4.7, delays=0/0.12/0/4.6, dsn=5.1.1, status=bounced (user unknown)
Apr 17 13:50:00 mx postfix/cleanup[11183]: 44kqWX1nDbz4xHTp: message-id=<44kqWX1nDbz4xHTp@mx.thatoneguy.com>
Apr 17 13:50:00 mx postfix/bounce[11229]: 44kqWR3dtQz4xHV2: sender non-delivery notification: 44kqWX1nDbz4xHTp
Apr 17 13:50:00 mx postfix/qmgr[9688]: 44kqWX1nDbz4xHTp: from=<>, size=4102, nrcpt=1 (queue active)
Apr 17 13:50:00 mx postfix/qmgr[9688]: 44kqWR3dtQz4xHV2: removed
Apr 17 13:50:06 mx postfix/pipe[11197]: 44kqWX1nDbz4xHTp: to=<klayne@thatoneguy.com>, relay=dovecot, delay=6, delays=0/0/0/6, dsn=5.1.1, status=bounced (user unknown)
Apr 17 13:50:06 mx postfix/qmgr[9688]: 44kqWX1nDbz4xHTp: removed
journalctl -u dovecot
-- Logs begin at Wed 2019-04-17 13:30:37 EDT, end at Wed 2019-04-17 13:50:06 EDT. --
Apr 17 13:31:12 mx systemd[1]: Starting Dovecot IMAP/POP3 email server...
Apr 17 13:31:15 mx systemd[1]: PID file /var/run/dovecot/master.pid not readable (yet?) after start.
Apr 17 13:31:15 mx dovecot[9119]: master: Dovecot v2.2.36 (1f10bfa63) starting up for pop3, imap, sieve, lmtp (core dumps disabled)
Apr 17 13:31:15 mx systemd[1]: Started Dovecot IMAP/POP3 email server.
Apr 17 13:47:42 mx dovecot[9265]: imap-login: Login: user=<klayne@int.thatoneguy.com>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=10322, secured, session=</o3Her2GULp/AAAB>
Apr 17 13:47:42 mx dovecot[9265]: imap(klayne@int.thatoneguy.com): Logged out in=154 out=1468
Apr 17 13:47:54 mx dovecot[9265]: imap-login: Login: user=<klayne@int.thatoneguy.com>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=10400, secured, session=<zV+He72GbLp/AAAB>
Apr 17 13:47:54 mx dovecot[9265]: imap(klayne@int.thatoneguy.com): Logged out in=219 out=1587
Apr 17 13:48:06 mx dovecot[9265]: imap-login: Login: user=<klayne@int.thatoneguy.com>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=10475, secured, session=<rGlJfL2Ghrp/AAAB>
Apr 17 13:48:06 mx dovecot[9265]: imap(klayne@int.thatoneguy.com): Logged out in=146 out=1490
Apr 17 13:48:22 mx dovecot[9265]: imap-login: Login: user=<klayne@int.thatoneguy.com>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=10582, secured, session=<pGIxfb2Gprp/AAAB>
Apr 17 13:48:23 mx dovecot[9265]: imap(klayne@int.thatoneguy.com): Logged out in=93 out=1128
Apr 17 13:48:34 mx dovecot[9265]: imap-login: Login: user=<klayne@int.thatoneguy.com>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=10656, secured, session=<gnPrfb2Gvrp/AAAB>
Apr 17 13:48:35 mx dovecot[9265]: imap(klayne@int.thatoneguy.com): Logged out in=93 out=1128
Apr 17 13:49:54 mx dovecot[9265]: imap-login: Login: user=<klayne@int.thatoneguy.com>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=11141, secured, session=<rsmhgr2GArt/AAAB>
Apr 17 13:49:54 mx dovecot[9265]: imap(klayne@int.thatoneguy.com): Logged out in=999 out=1739
I'm open to any other suggestions.
Thanks.