Email Deliverability: Why Your Emails Land in Spam (And How to Fix It)

Email deliverability is the measure of whether your emails actually reach your subscribers' inboxes — as opposed to landing in spam, promotions, or simply bouncing.

You can write a perfect email with a great subject line and a compelling offer. But if it goes to the spam folder, none of that matters. Deliverability is the invisible infrastructure that makes everything else in email marketing work.

This guide explains what deliverability is, what affects it, how to know if you have a problem, and what to do about it.

How Email Deliverability Works

When you send an email, it doesn't go straight to your subscriber's inbox. It travels through a chain of decisions:

Your email leaves your sending server (your email platform's infrastructure)
The receiving mail server (Gmail, Outlook, Yahoo, etc.) evaluates the email
The receiving server checks dozens of signals to decide: inbox, spam, or bounce?
The email is routed based on the evaluation

This evaluation happens in milliseconds. The signals being checked include your domain's reputation, your IP address's sending history, authentication records, content analysis, and subscriber engagement with your past emails.

If your emails are going to spam, it means these checks are returning negative signals. Here's which ones matter most.

The Main Factors That Affect Deliverability

1. Email Authentication (DKIM, SPF, DMARC)

These are DNS records that prove to receiving servers that you are who you say you are — and that your email platform is authorized to send emails on your behalf.

SPF (Sender Policy Framework): Lists which servers are allowed to send email from your domain.

DKIM (DomainKeys Identified Mail): Adds a cryptographic signature to your emails that verifies they haven't been tampered with in transit.

DMARC (Domain-based Message Authentication, Reporting, and Conformance): Tells receiving servers what to do when an email fails SPF or DKIM checks.

If these aren't set up, email providers treat your emails with suspicion. Setting them up correctly is the single most important technical step you can take for deliverability.

Most modern email platforms walk you through this setup. It involves adding a few DNS records through your domain registrar — takes 15–30 minutes and makes a dramatic difference.

2. Sender Reputation

Every IP address and domain that sends email has a reputation score maintained by email providers. This score is based on your sending history: bounce rates, spam complaint rates, engagement rates, and sending volume consistency.

Reputation is earned slowly and lost quickly. A sudden spike in spam complaints (someone marks your email as spam) or a high bounce rate (sending to invalid addresses) can tank a good reputation in days.

How to protect it:

Never send to purchased or scraped lists
Remove bounced addresses immediately
Monitor your spam complaint rate (keep below 0.1%)
Warm up new IP addresses gradually rather than sending full volume from day one

3. Subscriber Engagement

This is increasingly the biggest factor. Gmail, in particular, monitors how subscribers interact with your emails. Emails from senders whose previous emails were opened, replied to, and clicked are more likely to reach the inbox. Emails from senders whose previous emails were deleted unread, marked as spam, or ignored are increasingly filtered.

The implication: List quality matters more than list size. Sending to engaged subscribers protects your deliverability. Sending to disengaged subscribers harms it.

Practical actions:

Segment your list and send primarily to engaged subscribers
Run re-engagement campaigns for inactive subscribers
Remove or suppress subscribers who haven't engaged in 6+ months

4. Bounce Rate

A bounce is when your email fails to deliver. There are two types:

Hard bounces: Permanent delivery failures — the email address doesn't exist, the domain is inactive. These addresses should be removed immediately and never emailed again.

Soft bounces: Temporary delivery failures — the recipient's mailbox is full, the server is temporarily down. These usually resolve themselves, but repeated soft bounces should eventually be suppressed.

A bounce rate above 2% signals to email providers that you're sending to a stale or unverified list. Keep yours below 0.5%.

5. Spam Complaint Rate

When a recipient marks your email as spam, it counts against your sender reputation. Gmail considers a complaint rate above 0.1% problematic. Above 0.3% and your emails will start landing in spam folders systematically.

The main causes of spam complaints:

Emailing people who forgot they signed up (long time since last email, unclear sender name)
Sending emails people don't find valuable
Making it difficult to unsubscribe (so they mark as spam instead)
Emailing people who didn't actually opt in

Prevention: Use double opt-in. Make unsubscribing effortless — a single click, no login required. Send consistently so subscribers remember who you are. Include your business name in the "From" field.

6. Email Content

Content analysis catches obvious spam signals: certain phrases, excessive exclamation marks, all-caps subject lines, suspicious link patterns. Modern spam filters are sophisticated — they don't just keyword-match, they analyze the overall quality and legitimacy of the content.

Best practices:

Maintain a reasonable text-to-image ratio (not just one big image)
Include plain-text versions of all emails
Avoid URL shorteners in emails (they obscure destinations and are associated with phishing)
Don't use misleading subject lines
Include your physical address in the email footer (required by CAN-SPAM)

How to Diagnose Deliverability Problems

Check your open rates by email client. A sudden drop in Gmail opens specifically, while Outlook opens remain stable, suggests Gmail has started filtering your emails. This is usually an engagement signal problem.

Test your emails before sending. Services like Mail-Tester.com analyze your email and authentication setup and give you a score with specific recommendations. Running one test per month or before major sends is good practice.

Monitor your bounce and complaint rates in your email platform. Most platforms surface these metrics prominently. If your bounce rate spikes after adding a batch of contacts, those contacts are the problem.

Check blocklists. If you've had a major spam complaint event, your domain or IP may have been added to a blocklist. MxToolbox.com lets you check dozens of blocklists simultaneously. Being listed usually requires contacting the blocklist operator to request removal.

Send a test to Gmail and Outlook. Before a major campaign, send a test to accounts on both platforms and check where it lands — inbox, promotions, or spam.

Setting Up Authentication (Step-by-Step)

Most email platforms (including Flomailr) provide a checklist or guided setup for authentication. The general steps:

In your email platform: Find the domain authentication settings. The platform will provide specific DNS records — typically two or three text (TXT) records.
In your domain registrar (GoDaddy, Namecheap, Cloudflare, etc.): Add the DNS records your email platform gave you. This usually takes 5–10 minutes.
Wait for propagation: DNS changes take 24–48 hours to fully propagate. Most platforms will confirm when they're verified.
Verify: Your email platform will confirm that SPF and DKIM are correctly configured. A green checkmark means you're good.

Custom sending domains also improve deliverability because they separate your reputation from the email platform's shared IP pool — you're building your own domain's reputation rather than sharing one with thousands of other senders.

The Deliverability Maintenance Checklist

Run through this monthly:

[ ] Remove hard bounces from your list
[ ] Check spam complaint rate (target: below 0.08%)
[ ] Review open rates for unusual drops
[ ] Run a re-engagement campaign for 90-day inactives
[ ] Test emails with Mail-Tester before major sends
[ ] Confirm authentication records are still valid (DNS settings sometimes get accidentally changed)

The Bottom Line

Deliverability is unglamorous infrastructure work. But it's what makes everything else you do in email marketing actually work. A 30% open rate with good deliverability is worth far more than a 50% "open rate" that includes emails going to Gmail's Promotions tab where most subscribers never look.

Get your authentication set up. Keep your list clean. Send consistently. Protect your sender reputation like the asset it is.

Flomailr includes DKIM signing for custom sending domains, automatic bounce handling, and unsubscribe management — the technical foundation for solid deliverability from day one.