Why do emails land in the spam folder - Online Help | Zoho Campaigns

Why do emails land in the spam folder

A lot of effort goes into planning and framing a marketing campaign. With a little more effort, you can ensure that these campaigns yield a higher ROI instead of ending up in the spam folder. In short, emails land in spam when the mailbox provider doesn’t trust the sender: weak authentication, a poor sender reputation, low recipient engagement, or unwanted content. This question has many answers, so we’ll go over the information you need to consider in order to keep your campaign out of the spam folder.

Emails landing in spam – An infographic,

Spam: An Introduction

Spam is unsolicited, irrelevant advertising blindly sent to as many email addresses as possible. There are a few reasons why spam is considered to be dangerous for recipients:
  • Spam emails may carry malware and viruses.
  • Spam emails are mainly used for phishing attacks i.e., to act like a genuine sender to extract sensitive information like phone numbers, bank account details, etc.
  • Spam emails consume a lot of bandwidth which may cause problems for those with download limitations.
Because of this, Internet Service Providers (ISPs) and Email Service Providers (ESPs) implement spam filters to protect users from spam emails. Unfortunately, many legitimate emails may wind up in the spam folder as well, because of certain things which can be easily overlooked. It is essential to take all steps to help your emails reach your recipients’ inbox instead of their spam folder.

Spam filters

Major providers like Gmail, Yahoo, Outlook, and Apple Mail all have spam filters built into their service. (AOL Mail now runs on Yahoo’s infrastructure, so it applies the same filters and rules.) Today these filters are machine-learning systems that score every message across dozens of signals rather than simple keyword lists. The three that carry the most weight are your sender reputation, recipient engagement, and email authentication.

Content still plays a part, but a smaller one than it used to. Filters do watch for patterns common in spam — promotional phrases such as “weight loss”, “FREE”, or “earn from home”, image-heavy messages with little text, and suspicious links — but a single trigger word will not send a well-authenticated, well-engaged sender to spam. Reputation and engagement matter more than wording.

These filters make determinations about an email based on the user’s engagement. If the user opens, clicks, replies, or adds a sender to the contact list, then the email is less likely to be categorized as spam. On the other hand, if the recipient never opens your email or deletes it without reading the content, then you are more likely to be classified as spam. Sending to people who stopped engaging months ago drags down these signals for everyone on the list, so prune inactive contacts regularly.

ISP filters

ISPs use similar mechanisms to ESP spam filters to judge the sender’s domain, and they are typically more powerful. When they notice a domain is constantly sending email spam, they can simply blacklist your domain, and any emails sent from that domain will be blocked. If this happens, monitor your reputation with the providers’ own tools — Google Postmaster Tools and Microsoft SNDS — request removal from any blocklist that has listed you, and fix the underlying cause (authentication, complaints, or list quality) before sending again.

Common mistakes to avoid

  • Emails to people who haven’t opted-in — Never try to email people who haven’t opted-in to your emails. Sending messages to uninterested people will degrade your domain’s reputation and could lead to blocking or blacklisting. Domain reputation is something that many customers share, so the impact on its reputation means many will suffer. We strongly recommend sending messages to double opt-in contacts only.

  • Using a purchased mailing list — It is best to avoid this practice for the good of your domain and business. The companies that sell contact lists generally don’t spend much effort to make sure their contacts are interested in their customers’ products. There may even be spam traps in these mailing lists. To learn more, see spam traps.

  • Unauthenticated domain — Authenticate your domain with SPF, DKIM, and DMARC. Authentication is no longer optional: since February 2024, Google and Yahoo’s bulk sender requirements require all three for anyone sending to their users, and Microsoft applies the same rules to Outlook, Hotmail, and Live.com from May 2025. Mail that fails authentication is filtered to spam or rejected outright, so publish valid SPF, DKIM, and a DMARC record (start with a policy of p=none, then move toward p=quarantine or p=reject).

  • No easy way to unsubscribe — Bulk senders must include a one-click unsubscribe option and honor requests within two days. When people can’t unsubscribe easily, they mark your emails as spam instead. Keep your spam complaint rate below 0.3% — ideally under 0.1% — as measured in Yahoo and Google’s sender tools; crossing that line is one of the fastest ways to get filtered.

  • Scofflaw marketers — Make sure all of your marketers adhere to all spam laws, including CAN-SPAM, GDPR, CASL, and more. If these laws are not followed, your domain is at risk of a degraded reputation, or even legal retribution.

  • Unsegmented, unkempt mailing lists — When your mailing list is old and unsegmented, there can be a lot of chaos and irrelevant addresses. Unclean lists lead to spam traps, bounces, and more complaints against your domain. Make sure you constantly check your list and keep it updated. Segmentation can help make things easier for marketers who want to contact specific groups.

  • Too much/too little contact — If you either flood your recipients’ inbox or abandon your recipients, you’ll wind up with more complaints and unsubscribes. Ask for their preferences when they sign up for your mailing list, and you can avoid too much or too little contact.