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ISPmanager 6 Host: Emails Marked as Spam – Troubleshooting Guide
4 min read
16.03.2026
If emails sent from your server are being marked as spam, the issue typically stems from email server misconfigurations, poor sender reputation, or missing DNS records. Here is how to resolve the problem.
For the four mail-records walk-through inside the panel, see Configuring Mail Records (MX, SPF, DKIM, DMARC) in ISPmanager. For the broader ISPmanager troubleshooting reference, ISPmanager 6: Common Problems and Solutions.
Common Causes for Emails Being Marked as Spam
Missing Email Authentication Records
SPF, DKIM, or DMARC records are not set up or incorrectly configured.
Blacklisted IP Address
Your server IP is listed on spam blacklists.
Poor Domain Reputation
Email volumes or complaint rates are affecting your domain reputation.
Spam-Like Email Content
Your emails contain content flagged as spam by filters.
Reverse DNS (PTR) Record Missing
ISPs may reject emails from IPs without proper reverse DNS records.
Step-by-Step Solutions
Verify and Set Up Email Authentication Records
Proper SPF, DKIM, and DMARC records improve email deliverability.
SPF Record
- Authorizes which servers can send emails for your domain.
- Example SPF Record:
v=spf1 a mx include:yourdomain.com ~allDKIM Record
- Digitally signs your emails, proving they are not tampered with.
- Steps in ISPmanager:
- Go to Mail Settings > Mail Domains > Enable DKIM.
- Add the DKIM public key generated by ISPmanager to your DNS as a TXT record.
DMARC Record
- Sets policies for how ISPs handle unauthenticated emails.
- Example DMARC Record:
_dmarc.yourdomain.com TXT "v=DMARC1; p=none; rua=mailto:reports@yourdomain.com"Check Your IP Reputation
- Use Online Tools:
- Delist Your IP:
- If your IP is blacklisted, follow the delisting instructions provided by the blacklist service.
Configure a Reverse DNS (PTR) Record
- Why It Is Important: A PTR record links your server IP to your domain name, enhancing trust.
- How to Configure:
- Contact your hosting provider to set the PTR record.
- Ensure the reverse DNS resolves to mail.yourdomain.com.
Avoid Spam-Like Content
- Email Content Best Practices:
- Avoid all caps in subject lines.
- Limit the use of spam trigger words (e.g., "free", "discount").
- Use a proper text-to-image ratio.
- Test Your Emails:
- Use Mail Tester to analyze your email content and configuration.
Monitor Email Sending Limits
- Check Email Limits in ISPmanager:
- Navigate to Mail Settings > Limits to ensure you are not exceeding hourly sending limits.
- Implement Rate Limiting:
- If sending bulk emails, ensure you stay within allowed limits.
Verifying the Fix
Send a Test Email
- Send an email to multiple email providers (e.g., Gmail, Yahoo) to check spam placement.
Check Email Headers
- Review the headers of sent emails to verify:
- SPF: pass
- DKIM: pass
- DMARC: pass
Monitor Feedback Loops
- Set up feedback loops (FBLs) with ISPs to receive reports of spam complaints.
Advanced Solutions
Use SMTP Relays
- Route your emails through trusted third-party SMTP relays (e.g., SendGrid, Postmark).
- Benefits:
- Improved deliverability.
- Avoid using your server IP.
Enable SpamAssassin
- Use SpamAssassin in ISPmanager to filter incoming spam and improve email reputation.
Example DNS Records for Reference
| Type | Name | Value |
|---|---|---|
| SPF | @ | v=spf1 a mx include:_spf.example.com ~all |
| DKIM | default._domainkey | v=DKIM1; k=rsa; p=MIGfMA0GCSq... |
| DMARC | _dmarc | v=DMARC1; p=none; rua=mailto:admin@example.com |
Emails marked as spam can negatively affect communication and reputation. By implementing proper DNS records, securing your server, and monitoring email practices, you can improve email deliverability with ISPmanager 6.
Almost always the missing piece is PTR (reverse DNS). Many providers (Gmail in particular) refuse mail whose forward and reverse DNS don't match. Run dig -x — if the response doesn't match your mail hostname, ask your VPS provider to set the PTR record; you can't set it yourself.
For a quick sanity check, yes — it surfaces missing SPF/DKIM/DMARC, suspicious content patterns, and IP blacklist status in one report. Don't treat it as authoritative for Gmail or Microsoft 365 specifically (they have their own tooling), but it catches 80% of issues in under a minute.
Each blacklist has its own removal form. Look up the listing on MXToolbox, click through to the blacklist, fill the delist form. You usually have to fix the root cause first (kill the spammer, secure the leaking script) — automated delistings re-list quickly if traffic patterns don't change.
Yes. Many receivers reject mail whose HELO/EHLO greeting is something like "localhost" or "server1" instead of a real FQDN. Set the system hostname to a proper FQDN (server.yourdomain.com) and configure Exim/Postfix to use it for HELO.
Error 550 SC-001: What It Means and How to Fix It
Error: 421 Too Many Connections (8) From This IP — Causes and Solutions
"EHLO not accepted from server!" — Complete SMTP Error Fix
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