Several factors may directly influence whether your email is delivered to a recipient's inbox or spam box.
Here are a few tips to help you prevent that:
Make sure the recipient has marked you as a safe sender and has added you to a whitelist or address book.
Use at least 400 characters of text.
You are using an online email service provider (such as Hotmail) instead of a personal domain (yourorganization.com).
If you plan on using the Communications feature to send attachments, send to 10 recipients or less at a time.