What can I do so that mailings sent through Civi are not marked as spam?
An alternative solution is to use a third party service to send the emails and not the web server. They'll be better placed to ensure your not marked as spam. Services such as CiviSMTP (Paid), Mandrill (Free to small lists or with Mailchimp plans otherwise Paid) and Sendgrid (Paid) can all be used. I would recommend this as a longer term solution rather then trying to manage your own reputation.