I am trying to auto send message to users email addresses after registering on the website. Haven send the message to the zoho web mail, the message is delivering late, and not only that the message has question mark ? in place of the image. When i search what that  means, i realize the mail i am sending is not sending properly due to perhaps security issues. How do i resolve this please ?

Because , the mails may not delivered to some mails and i have to take precautions.

Please i need your response as soon as possible. Thanks