Email that doesn't comply with the law.
Many countries have laws in place that have clear guidelines for email senders. The PECR, CAN-SPAM and CASL are common examples.
These laws outline the very basic requirements that must be included in every email such as identification information, unsubscribe mechanisms and expressed consent from your contacts that confirm that they have agreed to receive mail from you. Senders who don't follow these guidelines may be found liable, face prosecution and will likely also upset their email recipients.
Email campaigns that are the most successful have one thing in common, they are sent to contacts that have agreed to and are expecting to receive mail from that sender.
Emails with bad/suspect content (phishing, spam words)
If your subject line, content or format look or sound like spam or phishing emails then it's a lot more likely to end up in the spam folder or rejected by recipient servers outright. Any similarity to a company or service that does not belong to you can mean your mail gets stopped. In addition, spam trends change regularly, you may be using a particular phrase in your email one week and it might become a problem the next, so you need to review your mail format regularly to ensure that your mail is not resembling a known phishing or spam email.
Emails with mistakes
Double check your emails before you send. An email that is poorly formed, has spelling mistakes or incorrect data can really affect the success of your current and future campaigns.
Marketing email with attachments
Unfortunately even if you're sending legitimate attachments, your mail may automatically be filtered into the spam folder. Adding attachments are typical protocol for spammers and phishing attacks and many subscribers will not trust mail that includes them.