Yes. Your server is denying relay to you. Because:
smtpd_sender_restrictions = permit_mynetworks mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
So you have to add you IP range (the IP of your telnet client, for instance) to my_networks
mynetworks = 192.168.0.15 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
In case you telnet client's IP is 192.168.0.15
If your server is hosted on an ISP and you are using unknown dynamic IP range you will have to use SMTP authentication in your server. Or just leave your server as an open relay (which of course I wouldn't recommend).