only zen cart generated emails end up in customers spam folder

I made the plunge, purchased a new domain and installed Zen cart version 1.38 only issue I am having is when Zen cart generates any emails (such as welcome & new orders) they end up in the customers spam folder.

I tested the emails through my Gmail, outlook and Incredimail account and this does not happen, they arrive in the inbox as usual.... It only happens when the email is generated from my Zen cart. My email options are set as:

E-Mail Transport Method = PHP
E-Mail Linefeeds = LF
Use MIME HTML When Sending Emails = true
Send E-Mails = true
Email Archiving Active? = true
E-Mail Friendly-Errors = true
Emails must send from known domain? = Yes

Web and mail server host is fine with no problems on their end...

Any guidance or input would be greatly appreciated.

Maybe the IP address of your webserver is listed in a blacklist database somewhere, and your customers' email servers are flagging them as spam simply based on their origin?