Mail server problems fixed

Due to an unfortunate (human) error, the email addresses at the @debianbook.info domain have not been working properly. The problem has now been fixed. If you have sent mail to any address ending in @debianbook.info over the past weeks, I would appreciate if you were so kind as to resend it.

Thanks to Andreas Fester for going through the trouble of contacting me by other means to alert me to the issue.