The first thing you need to do is focus on having the right subject line. Putting symbols or capital letter to your subject line might take it directly to the spam folder.
Also, it is really important to always create a text version along with the HTML version. Always try to make the text version as similar as possible to the HTML version. spam filters compare both versions, and if they are different they tend to place it in the spam section.
The main reason why email providers will block your emails is because:
Spam filters rate your email as a potential spam email. In other words, they gather characteristics such as words on the content of , size and color of the font, if the message is all spam or not and then depending on such factors, they make the decision of fixing it or not.
The key to your emails reaching the inbox is:
List hygiene: Make sure people have opted in directly to you, to receive your newsletters, and not to a third party. This will avoid your emails being flagged as spam, and your server will be clean, so when you send out your emails it will reach the inbox.