Deliverability Monitoring: A Practical Guide

Why Monitoring Matters

Deliverability problems are silent killers. Your emails can start landing in spam, and the only signal you'll see is a gradual decline in open rates over weeks. By the time you notice, the damage to your sender reputation may take months to repair. Proactive monitoring catches problems early — often before they affect a single campaign.

The goal of deliverability monitoring is simple: know where your emails are landing (inbox, spam, or blocked) across the major inbox providers, and get alerted immediately when something changes.

Inbox Placement Testing

Inbox placement testing uses "seed lists" — a set of test email addresses spread across Gmail, Yahoo, Outlook.com, AOL, and other providers. You send your campaign to these seed addresses alongside your real subscribers, and then check where the email landed in each test inbox.

How Seed Lists Work

A seed list service provides you with a set of email addresses (typically 50-200) that are monitored accounts across major inbox providers. You add these addresses to your send list. After sending, the service checks each account and reports whether your email arrived in the inbox, spam folder, or didn't arrive at all.

Tools for Inbox Placement Testing

• GlockApps: Affordable and straightforward. Provides inbox, spam, and missing placement rates across major ISPs. Their free tier gives you a limited number of tests per month, which is enough for monthly campaigns. Paid plans start around $59/month for regular testing.
• InboxReady (by Sinch/Mailgun): More enterprise-focused. Includes seed testing plus reputation monitoring, blacklist checking, and authentication validation in a single dashboard.
• Litmus (Spam Testing): Primarily an email preview tool, but includes spam filter testing that checks your content against major spam filters. Not a true inbox placement test but useful for catching content-based filtering issues.

Run inbox placement tests before every major campaign and weekly for automated sends. The test should be sent from the same domain, IP, and infrastructure as your production emails to get accurate results.

Google Postmaster Tools

Google Postmaster Tools is a free dashboard provided by Google that shows you how Gmail views your sending domain and IP reputation. Given that Gmail represents roughly 30% of global email opens, this is the single most important monitoring tool available.

Setup

Setting up Google Postmaster Tools requires DNS verification of your sending domain:

1. Go to postmaster.google.com and sign in with a Google account.
2. Add your sending domain.
3. Google provides a TXT record to add to your DNS. This verifies you own the domain.
4. Once verified, data starts appearing within 24-48 hours (you need sufficient daily volume to Gmail for data to be shown — typically 100+ messages per day).

Key Metrics to Monitor

• Spam Rate: The percentage of your emails that Gmail users marked as spam. Google's stated threshold is 0.10% — your spam rate must stay below this. Above 0.30% puts you at serious risk of blocking. Check this metric daily.
• IP Reputation: Rated as High, Medium, Low, or Bad. This reflects the aggregate behavior of all email sent from your sending IP(s). If you're on a dedicated IP, this is entirely your responsibility. On shared IPs, it reflects the pool's behavior.
• Domain Reputation: Similar to IP reputation but tied to your sending domain (the From address domain). Domain reputation is increasingly more important than IP reputation because it follows you across IP changes.
• Authentication Rate: The percentage of your emails that pass SPF, DKIM, and DMARC. This should be at or very near 100%. Any drop indicates a configuration issue or an unauthorized sender using your domain.
• Encryption Rate: The percentage of your emails sent over TLS. Should be 100% with any modern ESP.

Yahoo Sender Hub

Yahoo (which also handles AOL Mail) offers a similar tool called Sender Hub at senders.yahooinc.com. It provides complaint rates, reputation scores, and authentication status for Yahoo/AOL recipients. Since Yahoo and Google jointly announced stricter sender requirements in 2024, monitoring both platforms is essential for high-volume senders.

Major Blacklists

Email blacklists (also called blocklists or DNSBLs) are databases of IP addresses and domains that have been identified as sources of spam. ISPs query these lists as part of their filtering decisions. Being listed on a major blacklist can cause immediate, widespread delivery failures.

The Most Impactful Blacklists

• Spamhaus (SBL, XBL, PBL): The most widely used and most impactful blacklist. If you're listed on Spamhaus, a large percentage of your emails will be blocked or spam-filtered globally. Check at check.spamhaus.org. Spamhaus also maintains the DBL (Domain Block List) which lists sending domains rather than IPs.
• Barracuda (BRBL): Commonly used by corporate email servers running Barracuda spam filters. If you send primarily to business addresses, a Barracuda listing will disproportionately affect your delivery.
• Invaluement (ivmSIP, ivmSIP/24): Focused on identifying snowshoe spamming and ESP-based spam. Many major ESPs check Invaluement. Listings can be harder to remove than Spamhaus because Invaluement requires evidence of genuine opt-in practices.
• SpamCop: Driven by user reports. Listings are typically short-lived (24-48 hours) but frequent listings indicate a pattern that other blacklists may pick up on.
• SORBS (Spam and Open Relay Blocking System): Checks for open relays, open proxies, and sources of spam. Less commonly referenced by major ISPs now but still checked by some corporate filters.

What to Do If You're Listed

Don't panic. First, identify the cause — usually a spam complaint spike, a botnet on your IP, or a spam trap hit. Fix the underlying issue (clean your list, secure your infrastructure, tighten your acquisition practices). Then follow the blacklist's removal process, which typically involves submitting a delisting request and demonstrating that the issue has been resolved. Spamhaus is usually the fastest to delist (hours to days) if the issue is genuinely fixed.

Bounce Management

Bounces are emails that failed to deliver. Proper bounce management is both a hygiene practice and a reputation signal.

Hard Bounces

A hard bounce indicates a permanent delivery failure — the address doesn't exist, the domain doesn't exist, or the recipient server permanently rejected the message. Remove hard bounce addresses immediately after the first occurrence. Continuing to send to invalid addresses is the fastest way to damage your sender reputation and get blacklisted.

Soft Bounces

A soft bounce is a temporary failure — the recipient's mailbox is full, the server is temporarily down, or the message was too large. Retry soft bounces up to 3 times over 72 hours. If an address soft-bounces consistently over multiple sends (5+ consecutive soft bounces), suppress it as though it were a hard bounce.

Bounce Rate Thresholds

• Under 2%: Healthy. Normal for well-maintained lists.
• 2-5%: Warning. Review your list sources and hygiene practices.
• Over 5%: Critical. Pause sending, clean your list, and investigate the source of bad addresses.

Feedback Loops

Feedback loops (FBLs) are programs run by ISPs that forward spam complaint notifications to senders. When a subscriber marks your email as spam, the ISP sends a notification to your registered FBL address, allowing you to suppress that subscriber.

Major FBL programs include: Microsoft SNDS/JMRP (for Outlook.com, Hotmail, Live), Yahoo CFL (Complaint Feedback Loop), AOL FBL, and Comcast FBL. Most ESPs register for these automatically and handle suppression, but if you're running your own infrastructure, you need to register directly.

Google does not operate a traditional FBL. Instead, they provide complaint rates through Postmaster Tools. This means you can see your Gmail spam rate but cannot identify which specific subscribers complained — another reason to monitor Postmaster Tools closely.

Weekly Monitoring Routine

Effective deliverability monitoring requires consistency. Here's a practical weekly routine:

1. Monday: Check Google Postmaster Tools dashboard. Review spam rate trend, domain reputation, and authentication rates for the previous week.
2. Monday: Run a blacklist scan across Spamhaus, Barracuda, and Invaluement. Use a multi-check tool like MXToolbox or MultiRBL to scan multiple lists at once.
3. Wednesday: Review bounce rates from the week's sends. Ensure hard bounces are being suppressed. Investigate any soft-bounce spikes.
4. Wednesday: Check FBL complaint rates. Investigate any campaigns that generated above-average complaints.
5. Friday: Review overall engagement metrics (open rates, click rates) for the week. A sudden drop in engagement across multiple campaigns is often the first sign of a deliverability issue.
6. Monthly: Run a full inbox placement test across all major ISPs.

Set up automated alerts where possible. Google Postmaster Tools doesn't offer native alerts, but you can use its API to build custom monitoring. Most blacklist checking services offer email alerts when a new listing is detected. Your ESP should alert you to bounce rate spikes automatically.

Deliverability monitoring isn't glamorous work, but it's the difference between catching a problem in hours versus discovering it weeks later after your sender reputation has taken lasting damage.