To keep your Email out of the spam folder you must use Email Authentication, DKIM (Domain Keys Identified Email) and SPF (Sender Policy Framework) authentication can help verify that email comes from a trusted sender. DKIM is a validation system designed to detect email spoofing. It provides a means for your mail server to check email that comes from a domain is authorized by that domain's administrators. The email includes a digital signature that your mail server uses to verify the senders public key in their DNS record.
SPF uses DNS records to prevent email spoofing. If an email message uses fake sender addresses, SPF uses the DNS record for the sender's domain to detect the modification in the email's header.