Sure you want your email to look good; full of bright imagery and pretty font, but don't let that get in the way of your objective... which is to give your audience some key information that is going to make them want to do something.
It is important to be aware of the items in your control, which could trigger the email to be sent to the recipients spam folder, so that you can avoid doing them. I will break these down below and whilst it is not a guarantee, it is a starting point to move forward with.
Things to avoid
Including certain trigger words in your subject line, such as 'offer', 'free' & 'opportunity'. Check the Ultimate List of Email Spam Trigger Words compiled by HubSpot for more
Having sentences within your email body all in CAPITAL LETTERS - don't do it! Why do you have the need to shout at your loyal audience?
Excessive use of exclamation points! - You should really only have one thing to emphasise, which is what the whole email is about, so don't use the exclamation point for every sentence or at all
Attachments - a bit old skool, but some people still try to do this. Just link out and get them on your website. Attachments scream virus to me. Even if that email gets to me, I am not going to open it
High use of images vs. text - common rule is to have a 60/40 split of text to image ratio. Nowadays I see emails which are fully images. Even if this does get through to the inbox, sometimes it won't load and so the recipient may end up looking at a blank email
Different coloured fonts and/or styles - stick to one colour and one style, keep it basic too. People are reading emails across all sorts of devices & email clients and so even if you test it, you just don't know how it may appear for everyone. My guidance would be to stick to things like Georgia, Verdana & Arial.
Here's the kicker, some email clients are getting smart. They are starting to learn if the user deletes emails without opening them from that sender frequently and so are starting to filter them out automatically based on that user behaviour. Moral of the story, make sure your emails are relevant & useful... starting with a compelling subject line.