Send an email from the VirtualBox CentOS 8 server using your Google account as the sending address and using the app password

# dnf install mailx

Send test email

# mail <email_address>
Subject: test
test
.

# tail -f /var/log/maillog

Connection timeout...

Email forwarding settings

setting file

/etc/postfix/sasl_passwd
[smtp.gmail.com]:587 <google account email address>:16-digit app password

# postmap /etc/postfix/sasl_passwd

setting file

/etc/postfix/main.cf
relayhost = [smtp.gmail.com]:587
smtp_use_tls = yes
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_tls_security_options = noanonymous
smtp_sasl_mechanism_filter = plain

# systemctl restart postfix

firewalld settings

# firewall-cmd --list-services --permanet
cockpit dhcpv6-client ssh
# firewall-cmd --add-service=smtp --permanent
Success
# firewall-cmd --add-service=smtps --permanent
Success
# firewall-cmd --list-services --permanet
cockpit dhcpv6-client smtp smtps ssh

Send test email

# mail <email_address>
Subject: test
test
.

Check the queue

# postqueue -p

(Reference) Delete queue

# postsuper -d ALL deferred