Emails from postfix to external address display sender as '[email protected]'

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http://www.linuxquestions.org – I'm betting this is a ridiculously stupid question. Here goes. I'm setting up a very basic personal mail server on Slackware64 13.37 using postfix (obtained from slackbuilds.org). I can send mail to [email protected] just fine from [email protected] using Outlook. Everything processes correctly. However, when sending mail *from* [email protected] using mutt on the mailserver, [email protected] receives the message fine, except that instead of the expected '[email protected]', the sender is displayed as '[email protected]'. Right now, /etc/postfix/mail.cf is mini (HowTos)