How to increase credit of our domain for spam filters.

We have a domain with almost three hundred email addresses.
Some spam filters including barracudacentral some times lists our emails as spam.
Even some email clients marks our emails as spam when one employee sends email to another employee within the same organization.
We have an email newsletter which is powered by mailchimp and we only send to those who have subscribed to our newsletter and confirmed their email addresses.
I am asking how can we increase our credit so that they do not mark our emails as spam. How to solve all these problems. i regularly check some blacklisting sites to know whether we are blacklisted or not.