Bulk email

What causes my email messages from realestate@esch.com sent to yahoo EMail accounts to go into the bulk mail folder?

Is it realestate in my email address or the embedded links to real estate listings?

Also, it appears that if I have a EMail witha WORD file attachment and EMail to a client with Yahoo Email address, then it goes to the bulk folder, while a file without an attachment goes to the InBox.
Some people that request I send Email to their Yahoo EMail address can not figureout how to view the bulk folder and/or always accept EMail from my EMail address.