Zoho hosted domain sent mails goes to Outlook clients's Junk folder.

Zoho hosted domain sent mails goes to Outlook clients's Junk folder.

Hi, the mails which is sent from Zoho hosted domain mails are going to Junk folder of Outlook Client users. Mails sent to Gmail, Hotmail and Outlook.com works fine and the only problem is with Exchange server users who use MS Outlook Client. My domain have SPF records of 3 mx records of zoho added. But do not know why this problem occurs. Will DKIM sgnature fix this issue? If yes could someone give me a brief guide add DKIM signature?