Why your e-mails go to spam

A customer says they never got your offer, invoices land in spam, replies do not come. There are usually several causes, and they can usually be found and fixed. Here are the most common ones – in the order worth checking.

1. Missing domain authentication (SPF, DKIM, DMARC)

Large mailbox providers now require authentication. Since 2024 Gmail and Yahoo require at least SPF or DKIM from all senders and all three mechanisms from bulk senders; Microsoft introduced similar rules in 2025. Unauthenticated messages are far more likely to end up in spam or be rejected.

• Check your domain in our domain test.
• Set up SPF with all senders, enable DKIM and add DMARC.
• A common case: mail from the mail server is fine, but invoices from the accounting system or messages from the website form leave from another server that is missing from SPF.

2. The server is on a blacklist

If spam was sent from your IP address in the past (e.g. from a compromised account or computer), it may be on one of the public lists (RBL) that mail servers use. Check your outgoing server's address with one of the online blacklist checkers. If it is listed, first remove the cause, then request delisting – most lists describe the procedure on their website.

3. Missing reverse DNS or a mismatched server name

• The sending server's IP address should have a reverse record (PTR) with the server name, e.g. mail.company.com, and that name should point back to the same IP.
• The server should introduce itself (HELO/EHLO) with the same name.
• PTR is set by your internet or server provider, not by your DNS administrator.

4. A compromised account

When an attacker gets the password to one mailbox and sends thousands of messages from it, the reputation of the whole domain and server suffers. Warning signs: complaints about messages you did not send, bounces from unknown addresses, an unexpectedly full queue on the server. Fix: change the password, enable multi-factor authentication, check the mailbox for forwarding rules.

5. Bulk mailings

• Send newsletters through a dedicated service, not from a regular mailbox – this separates the reputation of business mail from marketing.
• Every bulk message needs an easy unsubscribe; large providers require one-click unsubscribe from bulk senders.
• Only send to people who asked for it. “Report spam” complaints hurt reputation the most.
• Regularly clean the list of non-existent addresses.

6. Message content

Content matters less today than authentication and reputation, but it still counts:

• link shorteners and links whose text does not match the real address,
• a message made of a single image without text,
• risky attachments (password-protected archives, executable files),
• a subject in all capitals with exclamation marks.

How to find out why a message went to spam

1. Send a message to a Gmail or Outlook.com address.
2. Open it and show the original message / source (Gmail: “Show original”).
3. Find the Authentication-Results header. spf, dkim and dmarc should all say pass. Anything else shows where the problem is.

If you are stuck, write to us – we help Čistý mail customers with their domain setup.