Second mail server for my domain

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http://serverfault.com – I have to setup a mail server because my hosting account don't allow me to send over 250 mail/hour. I have a HP microserver at home and I want to use it to send the mails. I configured Virtualmin on it and i created the domain example.com in it (I have example.com registered with hosting on my hoster) but when I tried to send mail from outlook, my server send the emails, but gmail mark it as spam... I tried to add a dkim (maybe without success) to the dns, and I tried also to add an SPF record to my dns to allow my server to send mail like the other (the 'original' server where I have the si (HowTos)