550 5.4.1 Recipient Address Rejected: Access Denied (Microsoft 365) Fix
550 5.4.1 Access denied means the recipient's Microsoft 365 tenant does not know the address. What senders can do, and the MX fix if the domain is yours.
3 min read
Deliverability
Inbox placement, bounces, reputation, and authentication.
Showing 1–24 of 58
550 5.7.1 "Messages Weren't Sent ... Block List (S3150)": Outlook.com Fix
What the Outlook.com S3150 and S3140 bounce means, why it names your provider's network, who can request delisting, and what to check on your domain.
3 min read
550 5.7.26 Sender Is Unauthenticated: The Gmail Bounce and the Fix
Gmail rejects mail with 550 5.7.26 when neither SPF nor DKIM passes for your domain. What each line of the bounce means, the usual causes, and the fix.
4 min read
554 5.7.1 Relay Access Denied: What It Means and How to Fix It
Relay access denied means a mail server refused to pass your message on. In a mail app it is authentication switched off; for incoming mail it is a wrong MX.
4 min read
How to Check If an Email Address Is Valid Before You Send
What you can verify about an address without sending (format, domain, MX, disposable domains), why SMTP verification is unreliable, and what proves an address.
4 min read
Emails Going to Junk in Outlook and Hotmail: Why, and How to Fix It
Why Outlook.com and Hotmail file your mail as junk, Microsoft's 2025 sender requirements, SNDS and JMRP, and the Safe Senders fix for individual recipients.
4 min read
Add Mailchimp, HubSpot or Shopify to SPF Without Breaking It
How to add third-party senders like Mailchimp, HubSpot and Shopify to your SPF record safely, count DNS lookups, and stay under the 10-lookup limit.
4 min read
Dangling DNS and Subdomain Takeover: Why Old Records Hurt Email
Dangling DNS records and subdomain takeover explained: how leftover CNAME, MX and SPF entries from cancelled services let attackers send as your domain.
4 min read
DKIM Key Rotation: Why, How Often, and How to Do It Safely
DKIM key rotation for small businesses: why keys should be replaced, a realistic schedule, and a two-selector method so nothing bounces during the switch.
4 min read
DMARC Aggregate Report XML Fields Explained (No Tool Needed)
Read a raw DMARC aggregate report (rua) by hand: what each XML field means, which rows show spoofing versus a misconfigured sender, and how to open .gz files.
4 min read
DMARC p=none to Quarantine to Reject: A 30-Day Plan With Exact Records
Move your DMARC policy from p=none to quarantine to reject in 30 days without losing legitimate mail. Exact TXT record values for each stage and what to check.
4 min read
Email Encryption in Transit: What TLS and STARTTLS Protect and Do Not
Email encryption in transit explained in plain language: how TLS and STARTTLS protect mail between servers and apps, what they do not protect, and how to check.
4 min read
Envelope From vs Header From: Return-Path and MAIL FROM Explained
The envelope From (MAIL FROM, Return-Path) is where bounces go; the header From is what people see. Learn why they differ and which one SPF and DMARC check.
4 min read
Lookalike Domains and Typosquatting: Spot, Monitor, and Defend
Lookalike domains and typosquatting explained for small businesses: the variants attackers register, how to monitor for them, and practical defences that work.
4 min read
Protect Your Domain From Hijacking: Lock, 2FA, Auto-Renew, Privacy
Protect your domain from hijacking with four registrar settings: transfer lock, two-factor authentication, auto-renew with a valid card, and WHOIS privacy.
4 min read
security.txt, abuse@, postmaster@ and security@: RFC 2142 and 9116
Every domain should have working abuse@, postmaster@ and security@ addresses and a security.txt file. What RFC 2142 and RFC 9116 ask for and a 20-minute setup.
4 min read
What Is a DNS Blocklist (DNSBL) and How to Check If You Are Listed
A DNS blocklist (DNSBL) is a list of IPs or domains tied to spam that mail servers query over DNS. Learn how listings happen and how to check if you are listed.
3 min read
What Is a Feedback Loop (FBL) and What Is a Spam Complaint Rate?
A feedback loop reports back when a recipient marks your email as spam. Learn what an FBL is, how spam complaint rate is measured, and how to stay under 0.3%.
3 min read
What Is a Return-Path and How Do Bounces Find Their Way Back?
The Return-Path is the address that receives bounces for an email. Learn what a Return-Path is, how it is set from MAIL FROM, and how bounces get back to you.
3 min read
What Is a Spam Score and How Do Email Filters Decide?
A spam score is the total of many small tests a filter runs on an email: content, reputation and authentication. Learn what a spam score is and what moves it.
4 min read
What Is ARC (Authenticated Received Chain)? Forwarding and SPF
ARC lets a forwarder or mailing list record and sign the authentication results it saw. Learn what ARC is, why forwarding breaks SPF, and how to read it.
4 min read
What Is IP Warming and Does a Small Business Need It?
IP warming means raising sending volume from a new IP gradually so receivers build trust. Learn what IP warming is and when a small business can skip it.
4 min read
What Is List-Unsubscribe? One-Click Unsubscribe (RFC 8058) Explained
List-Unsubscribe is a header that lets mail apps show an unsubscribe button. Learn how RFC 8058 one-click works and why Gmail and Yahoo require it.
3 min read
What Is Sender Reputation? Domain vs IP and How It Is Built
Sender reputation is the trust receivers assign to your sending domain and IP from past behaviour. Learn domain vs IP reputation and how to build it.
4 min read
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