454 4.7.1 Error Using Thunderbird with Postfix/Dovecot

I'm working through https://www.linode.com/docs/email/postf … -and-mysql">https://www.linode.com/docs/email/postfix/troubleshooting-problems-with-postfix-dovecot-and-mysql since the guide at https://www.linode.com/docs/email/postf … -and-mysql">https://www.linode.com/docs/email/postfix/email-with-postfix-dovecot-and-mysql didn't work for me. In fact, I'm in the "bottom-up" troubleshooting part of the page because the top-down part didn't help me.

I tested email send/receive using mailutils hosted ON my Linode server and both send and receive seem to work. The problem (454 4.7.1 Relay Access Denied) seems to happen only when I am trying to send email using my Thunderbird client remotely to another email service such as Gmail. But sending emails from Gmail to my virtual mailbox info@richardkarlschultz.com does work and Thunderbird can retrieve them.

I have a Charter high-speed cable Internet setup. I have a Linode with domain name richardkarlschultz.com. I have the following MX records:

Mail Server: mail.richardkarlschultz.com

Preference: 10

Subdomain: TTL: Default (5 mins)

I have an A record as follows:

Hostname: mail

IP Address: 45.79.128.130

TTL: Default (5 mins)

I am stuck on Step 19 of the "Virtual Domains and Users" section. It says to check if you can access email using IMAP and POP3, and I figured I should also be able to send email from an external email client (Thunderbird) at this point to test, but it's not working.

Here is my /etc/postfix/main.cf:

# Debian specific:  Specifying a file name will cause the first
# line of that file to be used as the name.  The Debian default
# is /etc/mailname.
#myorigin = /etc/mailname

smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
biff = no

# appending .domain is the MUA's job.
append_dot_mydomain = no

# Uncomment the next line to generate "delayed mail" warnings
#delay_warning_time = 4h

readme_directory = no

# TLS parameters
smtpd_tls_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key
smtpd_use_tls=yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache

# See /usr/share/doc/postfix/TLS_README.gz in the postfix-doc package for
# information on enabling SSL in the smtp client.

#smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
myhostname = mail.richardkarlschultz.com
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = localhost
relayhost =
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all
inet_protocols = all

#Virtual domains
virtual_mailbox_domains = richardkarlschultz.com, mail, mail.richardkarlschultz.com, localhost.richardkarlschultz.com
virtual_mailbox_base = /var/mail/vhosts
virtual_mailbox_maps = hash:/etc/postfix/virtual_users_list
virtual_minimum_uid = 100
virtual_uid_maps = static:5000
virtual_gid_maps = static:5000

When I configure my Outgoing Server (SMTP) in Thunderbird, I choose:

Server: mail.richardkarlschultz.com

Port: 25

Connection security: None or STARTTLS (either one)

Note that since I am only following along with the troubleshooting steps, I am NOT trying to do anything fancy here. I am just trying to figure out at the most basic level why I keep getting 454 4.7.1.