Postfix "warning: cannot get RSA private key from file"

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http://serverfault.com – I just followed this tutorial to set up a postfix mailserver with dovecot and mysql as backend for virtual users. Now I got the most parts working, I can connect to pop3 pop3s imap and imaps. Using echo TEST-MAIL | mail [email protected] works fine, when I log into my hotmail account it shows the email. It also works in reverse hence my MX entry for mydomain.com finally has been propagated, so I am being able to receive emails sent from [email protected] to [email protected] and view them in Thunderbird using STARTTLS via IMAP. Doing a bit more research after I got the (HowTos)