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.

EDIT:

As per FaheemMitha's advice, I tried telnet mydomain.com 587 from the client, and got No route to host in reply. I think this is because only port 25 is open in firewalld for smtp.

I therefore thought to try telnet from within the remote server mydomain.com. When I logged on to my remote server via ssh and typed telnet localhost 25, the result was:

Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 mydomain.com ESMTP Postfix  

This causes me to suspect that postfix is running on port 25, but that somehow it is not able to accept outside connections.

EDIT#2

As per @RedCricket's suggestion, I ran iptables -L. Since the results were verbose, I uploaded them to a file sharing site, which you can view by clicking on this link.

I also tried iptables --flush followed by firewall-cmd --reload, and then repeated the telnet and thunderbird tests from above, but I am still getting the connection timed out error.

What else can I try?

I uploaded the entire /etc/postfix/main.cf to a file sharing site. You can read it by clicking on this link.

EDIT#3

A valid email address someone.else@some_other_domain.com sends email to me@mydomain.com without problems. Therefore, as a test, I had my remote Thunderbird client try to send email to that someone.else@some_other_domain.com as part of the work documented above in this OP. This morning, I received a return to sender message in my Thunderbird as a result of the test email. I interpret this returned message to mean that at least one of my test messages from Thunderbird got into the SMTP on mydomain.com, but that mydomain.com was not able to look up or otherwise connect to some_other_domain.com. Here is the message:

This is the mail system at host mydomain.com.

I'm sorry to have to someone.elserm you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

The mail system

<someone.else@some_other_domain.com>: Host or domain name not found. Name service error for
    name=some_other_domain.com type=MX: Host not found, try again

Reporting-MTA: dns; mydomain.com
X-Postfix-Queue-ID: 2C915811BD1C
X-Postfix-Sender: rfc822; me@mydomain.com
Arrival-Date: Mon, 23 Feb 2015 16:46:34 -0500 (EST)

Final-Recipient: rfc822; someone.else@some_other_domain.com
Action: failed
Status: 4.4.3
Diagnostic-Code: X-Postfix; Host or domain name not found. Name service error
    for name=some_other_domain.com type=MX: Host not found, try again

ForwardedMessage.eml
Subject: key enclosed
From: me@mydomain.com
Date: 02/23/2015 01:46 PM
To:    someone.else@some_other_domain.com

this is the body of the email  

Thus, it seems that sometimes the connection from my remote devbox to mydomain.com is closed, and at other times, the connection from mydomain.com to the rest of the internet is closed.