There are a number of precautions you can take to make sure that your emails are not erroneously marked as spam. The most important is not to send people spam. Spam is, per Spamhaus.org, unsolicited bulk email. If you send email to multiple recipients who have not given you their permission to do so, you are sending spam.

The best way to avoid being marked as spam is to send emails to people if and only if they have verifiably demonstrated that they wish to receive your emails. Do not buy, rent, borrow, steal, or otherwise acquire lists of email addresses collected by other people. You have no way of knowing how those addresses were acquired or whether those recipients will have even the vaguest interest in your product.

Sending emails is so cost-effective that you may think sending 15 000 emails to reach 50 people is worthwhile. But having the other 14 950 people mark your email as spam will make it significantly more difficult for your future emails to reach inboxes.

In terms of content, here are a few dos and don'ts:

DO write like a human being. Normal, conversational copy will not trigger spam filters.

DON'T try to hide things in your email. An email that contains all of its copy in an image, or otherwise attempts to sneakily evade spam filters is more likely to be flagged as spam.

DO make sure that your HTML is well-designed. Poorly-coded HTML will look unprofessional to your recipients and suspicious to your recipients' spam filters.

DON'T get creative with spelling, grammar, or typography. Spammers often try to get around content filters by using characters that can be rec0gnis3d as letters by humans but not necessarily by computers. Unfortunately for spammers, humans are able to teach computers how to recognise this.

DO have an active and monitored email address to which recipients of your emails can reply. Some people would rather write a strongly-worded email than click on an unsubscribe link. If you don't remove those people from your list promptly, they are also the kind of people who will delight in making your life difficult.

DON'T try to hide your legally-necessary contact details and unsubscribe link by making them tiny or matched to the email's background colour. Spam filters look for this sort of thing. It may keep a few email addresses on your list, but none of them will feel particularly disposed to buy anything from you.

The best way to stay out of the spam folder is not to send spam. Spammers actively try to deceive or harm the recipients of their emails. You can easily distinguish your emails as legitimate by sending clearly-formatted emails which are easy to read and are only sent to people who want them. Make sure people know how to contact you about your emails, and that you have a functional unsubscribe link.

Treat your subscribers as correspondents rather than potential victims and you shouldn't have any problems.