Postfix in Docker container not forwarding virtual aliases

I am trying to configure Postfix for the first time. I do not need mailboxes, I want only virtual aliases, forwarding [email protected] --> [email protected]

My Postfix is running in a Docker container on a Digital Ocean droplet.

I have got as far as:

$ postalias -q [email protected]
[email protected]

...from inside the container, i.e. my /etc/postfix/virtual file is working.

Also, from outside the container on the droplet:

telnet example.com 25
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 example.com ESMTP Postfix (Ubuntu)

But from my own computer:

$ telnet example.com 25
Trying <droplet IP>...
telnet: connect to address <droplet IP>: Operation timed out
telnet: Unable to connect to remote host

I think this is expected and correct as a result of the mynetworks config setting (see below) which is as recommended by Digital Ocean - I don't want to host an 'open' SMTP relay.

I have an MX record for example.com. set up and I can ping example.com just fine and access websites on it. Also (from my computer):

$ host -t mx example.com
example.com mail is handled by 1 example.com.

So that looks right.

But if I send a test message to [email protected] nothing comes through and I don't see anything in Postfix logs either... I can't tell at what point it failed.

I am uncertain if the problem is in my Postfix configuration or in the routing into the container.

The container exposes port 25 (only) and is run via Fig with

ports:
  - "25:25"

From shell in the droplet:

$ netstat -tulpn | grep 25
tcp6       0      0 :::25                   :::*                    LISTEN      10680/docker-proxy

My /etc/postfix/main.cf has this in it:

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

smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
myhostname = example.com
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
mydestination = /etc/mailname, <container id>, localhost.localdomain, localhost, example.com
relayhost =
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all
virtual_alias_domains = example.com
virtual_alias_maps = hash:/etc/postfix/virtual
inet_protocols = ipv4

I'm not really clear of the distinction between myhostname (which was originally set to <container id>) mydestination and virtual_alias_domains

Updated

with output from http://mxtoolbox.com/SuperTool.aspx

Connecting to <server IP>

220 example.com ESMTP Postfix (Ubuntu) [733 ms]
EHLO MXTB-PWS3.mxtoolbox.com
250-example.com
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-STARTTLS
250-ENHANCEDSTATUSCODES
250-8BITMIME
250 DSN [714 ms]
MAIL FROM: <[email protected]>
250 2.1.0 Ok [722 ms]
RCPT TO: <[email protected]>
454 4.7.1 <[email protected]>: Relay access denied [715 ms]

MXTB-PWS3v2 4006ms

Update
As @masegaloeh helped me discover, my postfix server was basically working. I had two problems that confused things though:

1. Due to mistake in my Dockerfile I had /var/log/mail.log owned by root user... this is why it stayed empty. I didn't see any errors about it but basically rsyslog couldn't write to it. A chown syslog:adm /var/log/mail.log step fixed that, and I was able to see that indeed postfix was handling and forwarding mail to the alias.

2. I mistakenly believed I was able to telnet to other servers on port 25 from my laptop, just because I thought I'd done stuff in the past that would need that to work. But actually I can't. However I was able to telnet example.com 25 from another server so, again, stuff was actually working.

3. Sending mail to the alias from another server works, and comes through to my Gmail destination address.

4. It seems my problem is actually with Gmail... when I send the message to [email protected] from my Gmail account it does not show up. I since tried with aliases I had setup on another hosting... some work and some don't... leading to:

Conclusion:
it seems that Gmail will only accept mail for aliases that are configured under Settings > Accounts and Import > Send mail as ...unfortunately Gmail now requires you to specify a 3rd party SMTP server for it when setting up a new one, so it looks I will have to get to grips with TLS etc in my postfix install.