Understanding Email Deliverability: Common Causes of Low Deliverability

What is email deliverability? 📬

People often use "delivery" and "deliverability" as if they mean the same thing, but they don't, and the difference matters.

Email delivery is the purely technical handoff of your message: the receiving mail server accepts it, and at that point, delivery is technically complete. Your message has been handed off successfully.

Email deliverability (also called inbox placement) is what happens after that handoff. It's whether your message actually lands in the recipient's primary inbox, gets filtered into spam/junk, or gets silently dropped by the receiving system's filtering rules.

So a message can be "delivered" and still never be seen by anyone, because it landed in spam. That's the gap deliverability is all about closing.

How is deliverability measured?

There's no single deliverability "score." It's evaluated across a few different signals together:

• Inbox placement vs. spam rate — whether your messages land in the inbox or get routed to spam/junk. Mailbox providers track how often recipients mark your mail as spam, and that directly affects your reputation.
• Bounce rates (SMTP status codes) — rejections during or after the SMTP exchange. These fall into two buckets: temporary issues (4xx codes, like a full mailbox or a server that's momentarily busy) and permanent failures (5xx codes, like a mailbox that doesn't exist or an invalid domain). Learn more
• Sender reputation (health score) — mailbox providers assess the trustworthiness of your sending IP and domain based on things like spam complaints, invalid-recipient rates, and abusive sending patterns. This also includes your HELO/EHLO value, basically the hostname your server identifies itself with when it connects to another mail server. If that hostname looks inconsistent, mismatched, or suspicious, it can count against your reputation too.

Why there's no single fix 🔍

This is the part that trips people up most often: low deliverability is almost never caused by one thing. It's usually a combination of smaller factors stacking up, some technical, some behavioral. That means there's no universal switch to flip. Here are the areas worth checking:

1. Domain and IP authentication

Mailbox providers require a few standard protocols to confirm you are who you say you are:

• SPF (Sender Policy Framework) — authorizes specific IPs/servers to send on behalf of your domain. If the sending IP isn't listed, the check fails, and your email is more likely to be rejected or marked spam.
• DKIM (DomainKeys Identified Mail) — adds a cryptographic signature to your email header, proving the message wasn't tampered with and really came from your domain. A broken or missing signature makes filters treat the message with suspicion.
• DMARC — tells receiving servers what to do (none, quarantine, or reject) when SPF/DKIM checks fail, and requires that the visible "From:" domain lines up with what SPF or DKIM actually validated. If that alignment is missing, DMARC fails and your policy kicks in.
• MTA-STS and ARC — MTA-STS enforces encrypted transport (TLS) between servers, while ARC helps preserve authentication results when a message passes through an intermediary (like a mailing list) that modifies headers along the way.

2. Sending reputation (domain vs. IP)

Mailbox providers build up a reputation history for both your domain and your sending IP over time. Domain reputation tends to be the more stable, longer-term signal. Reputation can take a hit from high spam complaint rates or sending patterns that resemble fraud or malware.

3. Sudden spikes in sending volume 📈

Mailbox providers pay close attention to consistency. If a domain or mailbox that normally sends a small, steady volume suddenly jumps to sending hundreds or thousands of emails, that spike looks a lot like spam behavior, even if the content is completely legitimate. This is exactly why gradual warm-up matters: ramping volume up slowly gives providers time to build trust in your sending pattern, instead of flagging you as a risk the moment volume jumps.

4. Blacklists and blocklists 🚫

If your domain or IP ends up on a blacklist (like Spamhaus, SURBL, or Barracuda), a lot of mailbox providers will filter or reject your mail outright, regardless of how well everything else is configured. Getting listed usually comes from a combination of the other factors on this list (spam complaints, bad list hygiene, sudden volume, etc.), so it's often a symptom rather than the root cause, but it's worth checking directly if deliverability drops suddenly.

5. Content and formatting issues

Spam filters also look at how your message is structured:

• Line length — email headers and body lines should stay under 78 characters and must never exceed 998, per standard internet formatting rules. Going over can corrupt the message in transit or trigger spam filters.
• Character encoding — sending non-ASCII characters without properly negotiating the right transport encoding can result in garbled content, which filters don't like.
• Malformed headers — missing a valid From, Date, or Message-ID header (or having duplicate From fields) violates email standards and often gets messages rejected outright.
• Images and GIFs / image-to-text ratio — emails that are mostly one big image with little to no real text (or oversized GIFs) are a classic spam pattern, since spammers often use images to dodge text-based filters. A heavy image-to-text ratio can trigger filters even if the content itself is fine.
• Excessive links or link shorteners — too many links, or links routed through URL shorteners, can look suspicious to filters since shorteners are commonly abused to mask malicious destinations.
• Spam trigger words/phrases — certain words and phrasing patterns (aggressive sales language, ALL CAPS, excessive exclamation marks!!!, "FREE", "act now", etc.) are still weighed by many filters, though modern filters rely far less on this than they used to and lean more on reputation/engagement signals.
• Large or unusual attachments — attachments, especially executable-style file types or unusually large files, can trigger spam or security filters outright.
• Broken HTML — malformed HTML (unclosed tags, broken CSS) can cause rendering issues that some filters interpret as a red flag.

6. List quality and recipient engagement

• Invalid recipients — repeatedly sending to addresses that don't exist or are disabled damages your reputation fast. Keeping your list clean and verified matters a lot here.
• Hard-to-find unsubscribe options — if someone can't easily unsubscribe, they'll hit "spam" instead, and that's much worse for your reputation than a normal unsubscribe. Supporting one-click unsubscribe (List-Unsubscribe / List-Unsubscribe-Post headers) gives people an easy way out that doesn't hurt you.

7. Provider-specific policies

Gmail, Outlook, Yahoo, and others each run their own internal filtering logic, rate limits, and blocklists. Sometimes a rejection (like a "550 mail rejected per local policy") comes down to a provider-specific rule rather than anything universal.

A common misconception: Google Postmaster Tools 🙏

This one comes up a lot, so it's worth spelling out clearly: Google Postmaster Tools (GPT/GPM) is a reporting dashboard, not a deliverability fix.

It collects and displays signals Google already has about your domain, your spam complaint rate, authentication status (SPF/DKIM/DMARC), and, since a 2026 update, plain-language compliance and deliverability verdicts, but it doesn't generate or influence any of those signals itself. Connecting your domain to Postmaster Tools won't improve your deliverability on Gmail (or Outlook, Yahoo, or any other provider). It just gives you visibility into what's already happening, so you can act on it.

The same logic applies to any similar postmaster/reputation dashboard: it's a window, not a lever.

What actually moves the needle

Your real-world sending practices are what shape your reputation. To improve deliverability, the levers that actually matter are:

• Correctly configured SPF, DKIM, and DMARC
• A clean, engaged, verified recipient list
• Formatting that follows standard email specs
• Honoring unsubscribe requests promptly

Diagnosing a deliverability issue

If you're troubleshooting a specific problem, the most useful clue is usually the exact SMTP reply code or Enhanced Status Code in your bounce message (for example, codes like 5.1.1 for a bad address, 5.7.1 for a policy rejection, or 5.5.2 for a syntax error). These codes point to the actual reason a provider rejected or filtered your message, rather than guessing.

Since low deliverability is usually a mix of factors rather than one clear culprit, if you're seeing drops and aren't sure why, it's worth having someone take a closer look at your specific setup.