Email security guides
Practical, technical articles on spam, phishing, blacklists, SPF/DKIM/DMARC and data protection, written from the perspective of a team that runs mail infrastructure every day.
To diagnose a mail problem quickly, start with our free tools, then follow the guide that matches. If you want to protect your inbox, begin with what is a spam filter? If the mail you send isn't reaching people, see why are my emails going to spam? and SMTP error codes.
Core guides
The fundamentals of spam filtering, email security and deliverability.
What is a spam filter, and how does it work?
What is a spam filter and how does it work? Filtering layers, client vs. server vs. cloud vs. gateway filters, quarantine, false positives, and how to choose one.
What is an email security gateway?
What is a secure email gateway and how does it work? Mail flow, MX-based setup, what it filters, and how it works alongside Microsoft 365 and Google Workspace.
How to stop spam emails: 8 methods for businesses
How to stop spam emails at work: blocking senders in Outlook and Gmail, server-side filters, an email gateway, SPF/DMARC and staff awareness, step by step.
Why are my emails going to spam? Causes and fixes
Emails landing in spam at Gmail, Outlook or Yahoo? Fix SPF, DKIM and DMARC, blocklists, reverse DNS, content and complaint rates, with the 2024-2025 sender rules.
SPF, DKIM and DMARC explained
What SPF, DKIM and DMARC do, how they differ, how they work together, and how to add them to your domain step by step, with example DNS records.
Mail server blacklisted? What to do, step by step
Is your mail server's IP on a blacklist? How to confirm it, stop the source, get delisted from Spamhaus, Barracuda, SpamCop, UCEPROTECT and Microsoft.
How to spot a phishing email: 7 warning signs
How to tell a phishing email from a real one: 7 common warning signs, what to do with a suspicious message, and a checklist you can share with your team.
Email security and GDPR: a practical guide for businesses
What GDPR expects from business email security: Article 32 measures, common email breaches, the 72-hour notification rule and a practical checklist.
For resellers, hosting providers and MSPs
Sell spam filtering to your customers under your own brand. Reseller program
Antispam reseller programs: a guide for hosting providers and MSPs
What an antispam reseller program is, who it suits and how it earns: per-domain pricing, reseller panel, API and the application process, step by step.
What is a white label spam filter? Selling antispam under your own brand
What a white label spam filter is and how it works: the benefits of selling antispam under your own brand, what to watch for and how to choose a provider.
How to sell spam filtering to your hosting customers
A practical guide for hosting providers: packaging, pricing, pitching and setting up spam filtering as an add-on service your customers actually buy.
WHMCS spam filter integration: selling antispam with the reseller API
Create a spam filtering product in WHMCS and automate domain provisioning, cancellation and MX checks with the Spambil reseller API. Example requests and flow.
SPF, DKIM and DMARC setup by platform
DNS records and steps for the mail service you use. All platforms
Google Workspace
SPF, DKIM and DMARC records for Google Workspace, with step-by-step setup.
Microsoft 365
SPF, DKIM and DMARC records for Microsoft 365, with step-by-step setup.
Yandex 360
SPF, DKIM and DMARC records for Yandex 360, with step-by-step setup.
Zoho Mail
SPF, DKIM and DMARC records for Zoho Mail, with step-by-step setup.
cPanel hosting
SPF, DKIM and DMARC records for cPanel hosting, with step-by-step setup.
Blacklist removal
Every blocklist has its own rules. Step-by-step removal for the list you're on.
How to get removed from Spamhaus
Listed by Spamhaus? The difference between SBL, XBL, PBL, CSS and DBL, how to get removed from each, and how to avoid being listed again.
How to get removed from the Barracuda blocklist (BRBL)
Is your IP on the Barracuda Reputation Block List? Why it happens, how to submit a free removal request at barracudacentral.org, and what to fix first.
How to get removed from SpamCop
Listed on SpamCop? How SpamCop works, when a listing expires on its own, and what you need to fix so your IP isn't listed again.
How to get removed from UCEPROTECT
What UCEPROTECT Level 1, 2 and 3 mean, when a listing expires on its own, why you don't need paid express delisting, and what to do instead.
How to get unblocked by Microsoft (Outlook.com, Hotmail, Microsoft 365)
Mail not reaching Outlook.com, Hotmail or Microsoft 365? What S3150, S3140, 5.7.606 and 5.7.515 mean, and how to use Microsoft's free delisting options.
SMTP error codes
What the code in a bounced email means and how to fix it. All codes
550 5.1.1 error
User unknown (recipient does not exist)
550 5.4.1 error
Recipient address rejected: Access denied (Microsoft 365)
550 5.7.1 error
Message rejected: policy or permission failure
550 5.7.23 error
SPF validation failed
550 5.7.26 error
Unauthenticated sender or DMARC policy rejection
554 5.7.1 error
Transaction failed: relay denied or blocked
421 4.7.0 error
Temporary rejection: rate limit or reputation
451 4.7.1 error
Temporary rejection: greylisting (try again later)
552 5.2.2 error
Recipient mailbox full