Published: 23.09.2026
A message that lands in the spam folder is not always the result of a technical error. Receiving servers decide with many signals, and the reputation of the sender is a major one. This article explains the usual reasons mail is filtered, how reputation builds up, what the authentication records do at a high level and how to keep your recipient lists clean.
Why messages end up in spam
- The sending address has a poor history of complaints, bounces or messages to spam traps.
- The message fails authentication checks, so the receiver cannot confirm who really sent it.
- The content looks like bulk marketing, with many links, capital letters and misleading subject lines.
- The sending pattern changes abruptly, such as thousands of messages from a new address overnight.
- Recipients marked earlier messages as spam, which trains the filters against you.
Reputation builds slowly
Receiving servers keep track of the domains and IP addresses that send them mail. A steady pattern of expected messages to people who want them builds a good history, while a sudden burst to unknown addresses damages it. When you switch providers or start a large campaign, increase the volume gradually over several days or weeks. This is often called warming up, and it gives receivers time to see that your mail is wanted.
Authentication in plain words
Three DNS-based mechanisms help a receiving server check that a message is genuine. SPF lists the servers that are allowed to send mail for your domain. DKIM adds a digital signature, which proves that the message was not changed on its way. DMARC tells receivers what to do when these checks fail and can send reports back to you. You do not need to master every detail, but publish the records exactly as your mail provider gives them, because a small typo can block legitimate mail.
Keep your lists clean
- Send only to people who asked to receive the mail, and keep a record of their consent.
- Remove addresses that bounce permanently after the first failure.
- Stop sending to people who have not opened or clicked anything for a long time, and ask whether they still want the mail.
- Make unsubscribing easy, and honour requests quickly.
- Never buy address lists, because they often contain spam traps and invalid addresses.
Content and formatting
Write messages as a person would. Keep the subject honest and specific, use images and links in moderation, and show a clear sender name and contact details in the footer. A plain-text version helps some filters. Before a large send, check how the message looks on a phone and in a desktop mail client, because broken layouts make recipients suspicious.
Watch the signals
Read bounce messages and complaint reports regularly. A rising bounce rate usually points to a list problem, and complaints mean that recipients do not want the mail. If legitimate messages start to disappear into spam, check in this order: the authentication results of a test message, the sending volume and the content. Keep a note of every change, so that you can see which one made the difference.
Frequently asked questions
Will a dedicated IP address fix spam problems? Not by itself. A dedicated address can separate your reputation from other senders, but poor lists and weak content still cause filtering.
Why does my mail reach some providers but not others? Each receiving service runs its own filters and holds its own view of your reputation, so results can differ between them.
How many messages can I send safely? There is no fixed number that suits everyone. Increase the volume gradually, and let the complaint and bounce rates guide the next step.
