Because you do not allow Salesforce to send emails from your domain name.
Your domain is “xyz.com” and the SalesRep’s email is “email@example.com”. An automated process in Salesforce sends an email to one of your customers. So, the email is sent by Salesforce but the from-address of the email is “firstname.lastname@example.org”, this is seen as SPAM as the sender servers are not from the domain “xyz.com”. If anybody was able to send emails from your domain … that would make phishing very easy.
Now, how do you allow Salesforce to send emails from your domain?
The Salesforce Admin must work together with the Domain Admin to set this up in just 10mins. All that is required is:
DKIM (Domain Keys Identified Mail): Article
SPF (Sender Policy Framework): Article