postfix smtp connection timed out, why?

A CentOS 7 web server has postfix, dovecot, and mailx installed. I have been able to make an IMAP connection to the server in order to read inbox mail using a remote Thunderbird client, but I am not able to make an SMTP connection to send email from Thunderbird. When I do forensics, I discover that the attempted SMTP connection times out. How can I resolve this problem of the connection timing out, so that I can send email from Thunderbird through the server?

My forensics so far have resulted in:

Typing hostname in the terminal at the server returns mydomain.com.

nano /usr/lib/firewalld/services/smtp.xml indicates the smtp port is 25

The smtp service is activated in the public zone because firewall-cmd --list-all results in:

public (default, active)
  interfaces: enp3s0
  sources: 
  services: dhcpv6-client imaps openvpn smtp
  ports: 
  masquerade: yes
  forward-ports: 
  icmp-blocks: 
  rich rules: 

But when I try to telnet from my devbox to the remote CentOS 7 server, I get the following results. Typing telnet mydomain.com 25 resulted in:

Trying my.SERVER.ip.addr...
telnet: connect to address my.SERVER.ip.addr: Connection timed out

Then typing telnet smtp.mydomain.com 25 resulted in:

Trying my.SERVER.ip.addr...
telnet: connect to address my.SERVER.ip.addr: Connection timed out

Also, typing openssl s_client -CApath /etc/ssl/certs -starttls smtp -port 25 -host smtp.mydomain.com results in:

socket: Connection timed out
connect:errno=110

Similarly, typing openssl s_client -CApath /etc/ssl/certs -starttls smtp -port 25 -host mydomain.com also resulted in:

socket: Connection timed out
connect:errno=110

I typed nano /etc/postfix/main.cf to start to examine the config, but did not find anything related to ports.