Every mail servers has its built-in anti-spam filters. Your emails are not delivered or filtered (landed at spam/junk folder) because your email message content is detected or recognized as spam by email providers or anti-spam organizations such as SpamAssasin.
You may refer to the tips below on how to improve inbox rate:
i) Get permission
It is the MOST IMPORTANT and BASIC measure to take in order to ensure good deliverability. Only send emails to those addresses that subscribed to your newsletter (people go to your website and enter their email addresses to get the specific information from you).
ii) Subject and message content
Do not use keywords with high spam score either in the email subject or in the body of the message (such as "FREE", "click here!", "buy now!" and etc). You can use the spam score checker function that is provided by emailblasting software.
iii) Unbalanced message content
If your email message is full of images and almost no text, most probably your email will land in junk/spam folder. Anti-spam filters become suspicious when you are sending high volume emails with images. We may suggest you to use links to files hosted on a web server.
iv) Avoid attachments
It is better not to attach any files to your emails. Please use links to files hosted on a web server to avoid your email marked as spam.
v) Don’t use generic email address
Instead of using a free email address such @hotmail.com or @gmail.com as your “from” email, you should use an registered domain as email address. Addresses hosted at @hotmail.com or @gmail.com are commonly used by spammers.
vi) Include unsubscribe links
Frequently manage/update your mailing list. Provide “unsubscribe” links for your subscribers to remove their email address from your list if they wish to do so.
vii) Remove inactive subscribers
Delete your old & inactive subscribers from your list. Subscriber engagement plays a big role in email delivery. By focusing on your active subscribers, you can increase your overall delivery rates.
For more information please refer to the guidelines provided by email providers:
Yahoo Mail: http://help.yahoo.com/kb/index?locale=en_US&page=content&y=PROD_MAIL_ML&id=SLN3435&impressions=true
cPanel Support: http://forums.cpanel.net/f34/why-didnt-my-email-get-delivered-where-did-my-email-go-387312.html