Stephen - without looking at the log.....
If you are sending an email from a me.com account , you need to use the smtp server at me.com in your mail client options. If you use your servers smtp , icloud.com ( me.com) will consider this a relay attempt and reject it. with postfix logging set at debug, your mail.log will show you the error.
Try logging into icloud.com and send the mail from the http interface - it should all be good.