Postfix Connection refused (port 25)
The mail log shows that you CAN connect to your postfix server (on localhost). The postfix cannot connect to the external smtp server (mail.mydomain.com), and you can't connect with telnet either.
Perhaps you have a firewall (at your ISP) that blocks the outgoing smtp. This is common to block spam. For workaround you can use other mail server (gmail?) for relaying your e-mails, and connect to it on secure smtp (port 465 / 587) and authentication.
This is a great howto for this: https://www.howtoforge.com/tutorial/configure-postfix-to-use-gmail-as-a-mail-relay/