Pillar 009
ESP & Infrastructure
Every ship needs a sturdy hull. Understand how Email Service Providers, servers, and sending IPs keep your operation afloat. Learn how routing, throttling, and infrastructure choices influence deliverability.
Overview & Fundamentals
- What is an ESP (Email Service Provider)?
- What’s the difference between an ESP and an MTA (Mail Transfer Agent)?
- How does an ESP send emails?
- What is a mail relay?
- What’s the role of DNS in email delivery?
- What is an SMTP server and how does it work?
- What are incoming vs outgoing mail servers?
- What’s the difference between SMTP, IMAP, and POP3?
- What is a sending pipeline?
- How do ESPs queue and retry messages?
- What’s a message ID and how is it generated?
- How do ESPs track delivery and bounce events?
- What is shared vs dedicated infrastructure?
- How does a sending API differ from SMTP submission?
Domains, Subdomains & Routing
- Why do ESPs recommend using custom sending domains?
- What is a return-path or bounce domain?
- What’s the difference between envelope-from and header-from?
- How many sending domains should I use?
- Should I use subdomains for different email streams (marketing vs. transactional)?
- What is a click-tracking and open-tracking domains?
- How do I manage the reputation of multiple IPs/domains?
- What is a smart host or relay host?
- What is mail routing and how does it differ from forwarding?
- What’s the difference between internal and external routing?
- How can you test routing paths (traceroute, MX lookup)?
- What’s the impact of domain age and reputation?
- How do redirects and shorteners affect domain reputation?
Providers (ESP, SMTP, MTA)
Mail Transfer Agent (MTA) Configuration
IP Reputation & Management
- What is IP pooling?
- How do mailbox providers score IPs?
- How do ESPs pool IPs?
- How do IPs get delisted from blocklists?
- What is “cold” vs “warm” IP reputation?
- How does throttling prevent IP reputation loss?
- What’s the difference between temporary and permanent blocks?
- How to detect IP rotation abuse?
- What are best practices for maintaining IP reputation?
- How do tools like SNDS or Postmaster monitor IP health?
- Can IP reputation recover after a blocklist event?
- How do feedback loops influence IP reputation?
Sending Infrastructure & Queues
- What are email queues?
- What causes emails to get stuck in queue?
- What is a retry policy?
- What is a deferred message (4xx error)?
- How does retry timing affect delivery?
- What are backoff algorithms in MTAs?
- How does load balancing work for large ESPs?
- What’s a rate limit and how do ESPs manage it?
- What’s the difference between 421 and 451 errors?
- How can ESPs detect and pause bad senders?
- What is greylisting and how does it affect queues?
- How do queues interact with reputation systems?
- How do “burst sends” differ from throttled batches?
Reading Email Headers
- What is an email header?
- What is the difference between envelope and header?
- Why would I need to look at email headers?
- How do I view full email headers in different clients (Gmail, Outlook, etc.)?
- How to read and interpret raw headers?
- What key information can I find in email headers (e.g., Received, Authentication-Results, Message-ID)?
- What do “Received” headers mean?
- What is a “Return-Path”?
- What is “X-Mailer” or “User-Agent”?
- What are custom X-headers and why use them?
- How can headers help troubleshoot deliverability problems?
- What tools can help analyze email headers?
- What is ARC-Seal, ARC-Message-Signature, and ARC-Authentication-Results?
- What is “Authentication-Results” and how to read it?
- What is the difference between SPF and DKIM results in headers?
- How to detect spoofing from headers?
- What is “Received-SPF”?
- What are “List-Unsubscribe” headers?
- What is “Feedback-ID”?
Link & Image Infrastructure (CDNs, Trackers)
- Why use a custom domain for tracking links?
- How does click tracking work?
- How does link tracking domain reputation affect deliverability?
- What are redirect domains?
- What is link wrapping?
- How do ESPs count unique vs total clicks?
- What is pixel tracking for opens?
- How does MPP (Mail Privacy Protection) affect tracking?
- What are custom tracking domains and why use them?
- What are privacy implications of tracking?
- What are the risks of third-party link shorteners?
- How do anti-phishing filters inspect links?
- What are “safe link” rewriting systems (Microsoft ATP, Gmail Safe Browsing)?
- How can link patterns hurt deliverability?
- How can you disable link tracking selectively?
- What’s the difference between click tracking and engagement scoring?
- Should my link domain match my sending domain?
- Where should I host images for emails?
- What is a Content Delivery Network (CDN)?
- How do CDNs help with email image loading?
- How does Gmail's image proxy work?
Message Mechanics (MIME, Attachments, List-Unsub)
- What is MIME (Multipurpose Internet Mail Extensions)?
- What are MIME types?
- How are HTML and plain text emails structured using MIME?
- How do attachments work in email?
- Do attachments affect deliverability?
- What file types are risky to send as attachments?
- What is the List-Unsubscribe header?
- What's the difference between mailto: and http: List-Unsubscribe?
- What are Precedence: bulk and Auto-Submitted headers?
ESP Architecture & Scaling
- How do ESPs scale to send billions of emails?
- What are MTA clusters?
- What is sharding in message storage?
- How do ESPs distribute sending load?
- What’s the difference between horizontal and vertical scaling?
- How does message deduplication work?
- How do ESPs prevent cross-customer contamination (reputation-wise)?
- What is a sending node or data center?
- What are regional routing rules (GDPR, data residency)?
- What happens when an ESP data center goes down?
- How do ESPs manage backups and disaster recovery?
- What is HA (High Availability) in email delivery?
- What are ESP failover strategies?
Security & Infrastructure Hardening
- What are inbound vs outbound filters in ESPs?
- How do ESPs prevent account compromise or abuse?
- What is SMTP AUTH and why is it important?
- What is rate limiting per account or token?
- How do ESPs detect and block spam campaigns internally?
- What is anomaly detection in email traffic?
- What is encryption in transit (TLS) and at rest?
- How do ESPs handle sensitive data storage?
- What’s the difference between opportunistic and enforced TLS?
- What is DANE and how does it relate to MTA-STS?
- How do ESPs verify webhook and API integrity?
- How can senders audit security in ESP dashboards?
Troubleshooting & Diagnostics
- What are the most common ESP-related deliverability issues?
- How to interpret ESP bounce classifications?
- What does “temporary deferral” mean in logs?
- How to analyze delivery logs?
- How to isolate failures by domain or ISP?
- What tools can you use for SMTP debugging?
- What’s the difference between SMTP transcript vs event log?
- How do you test relay authentication?
- What’s the difference between 550 and 554 errors?
- What is message throttling and how to detect it?
- How do ESPs communicate delivery issues to users?
- What’s the difference between hard and soft failure codes?
- How to correlate ESP logs with Postmaster data?
ESP Switching & Migration
- How do you migrate from one ESP to another safely?
- What are risks during ESP migrations?
- How to move sender reputation between ESPs?
- What is IP/domain warming after migration?
- What’s the best way to export/import suppression lists?
- How do you transfer templates, automations, and preferences?
- How do ESPs differ in bounce classification?
- How long should you keep both ESPs active during transition?
- What’s the difference between full migration and split delivery?
- How to update DNS records during migration?
- What are signs that migration hurt deliverability?
Hybrid & Custom Infrastructure
- What is hybrid sending (ESP + custom MTA)?
- Why do some companies build their own mail servers?
- What are the benefits and risks of self-hosting email infrastructure?
- What’s the difference between Postfix, Exim, and PowerMTA?
- What is a “deliverability cluster”?
- How to integrate custom routing with ESP tracking?
- How do hybrid setups handle authentication?
- How do internal relay chains affect traceability?
- What’s the advantage of on-premise logs?
- How to prevent custom servers from becoming open relays?
- What is message queuing middleware (RabbitMQ, Kafka) in email pipelines?
- When does hybrid make sense vs commercial ESP?
Trust beats tactics.
Review My Emails · Email Almanac