Generally, that has nothing to do with framework/platforms like UNA and everything to do with the mail system/program used on the server.  It's usually an issue with newer domains and the more you send email that isn't flagged as spam, the less likely it will get a 'spammy' rating.  One hack I've used is to send email to my own Gmail, then report as 'not spam'.  Believe it or not, that often resolves the issue with Gmail. 

If all else fails, get the SMTP plugin and config to use mandrill or sendgrid.