Some emails going to customers' SPAM folder but not all?

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Hey folks,

 

I am having an issue with emails ending up in customers' SPAM folders.

 

When a client signs up for an account the NEW ACCOUNT INFORMATION email arrives in their Inbox just fine but the WELCOME and ORDER CONFIRMATION emails end up in the SPAM folders.

 

I own my own server so there is no data center to work with on that. I have tried to do rDNS stuff and have made sure we are not blacklisted but it still ends up that way no matter what.

 

I am not 100% certain I am setting up the DNS mail correctly but I did a lot of research on a lot of various sites so I would think they were set up right.

 

I have also switched WHMCS over to SMTP with the same results as the PHP mail.

 

I also changed the SUBJECT in the email templates to see if that was it but that is also a no go.

 

Any ideas?