How to Fix Email Deliverability Issues (Guide 2025)

Every email you send is evaluated based on three main criteria: technical configuration, domain reputation, and recipient engagement. When any of these signals fail, messages get filtered to spam or disappear entirely, hurting opens, replies, and revenue.

The root causes are often attributed to missing authentication protocols, an unestablished or weak domain, poorly structured content, or outdated mailing lists. 

Fortunately, most deliverability problems trace back to a handful of recurring issues and the fixes are straightforward once you know where to focus.

7 Email Deliverability Issues and How to Fix Them

Email deliverability failures in cold outreach stem from a small set of recurring issues that Gmail and Outlook consistently track. 

If you’re sending outreach emails to Google Workspace or Office 365 inboxes, here’s what typically goes wrong, and how to fix it before your domains get burned.

Poor sender reputation (and how to repair it)

Your sender reputation is like a credit score for your domain. And in cold outreach, it depends entirely on engagement. Gmail and Outlook don’t care whether prospects opted in; they care how people react. One spam complaint outweighs ten opens. Ignored emails, high bounce rates, or low reply rates all send negative signals that drag your reputation down. 

When that happens, even a solid copy won’t save you. Suddenly, open rates dip, replies vanish, and your sequences land in junk.

How to fix it:

Slow down and rebuild trust. Drop daily volume, focus on your most engaged contacts, and let positive engagement accumulate. 

Run MailReach email warm-up. It simulates authentic interactions across Gmail and Outlook that accelerates this recovery. 

Also, check Google Postmaster’s domain reputation curve. When it climbs back from “low” to “medium” or higher, you know you’re on the road to repair.

Authentication errors (and how to validate them)

In 2025, missing or broken authentication is one of the fastest ways to land in spam. Gmail and Yahoo bulk-sender rules require SPF, DKIM, and DMARC to pass. If they’re missing or misaligned, your emails look spoofed and get blocked. SPF fail or DKIM fail in bounce logs is the giveaway.

Some teams try to patch this with custom SMTP servers, but that doesn’t work. Deliverability is judged at the domain level, not the sending pipe. If DMARC doesn’t align, Gmail will still dump you in spam, no matter what SMTP you use.

How to fix it:

Make sure SPF, DKIM, and DMARC are published correctly and aligned with the visible From domain. Start DMARC in monitoring mode (p=none) so you can see where failures come from, then move to stricter enforcement once everything is aligned. 

Use MailReach’s SPF and DKIM checkers too to double-check whether you’ve set those records up correctly. 

Errors in Reverse DNS and IP setup (and what to do about them)

Reverse DNS (PTR) is like a caller ID for your sending server. 

When Gmail sees a connection from your IP, it checks whether that IP correctly maps back to the domain claiming to send the mail. If the mapping doesn’t match, Gmail treats the send as suspicious, like a phone call that says it’s your bank but shows up as an unknown number. Even if your SPF and DKIM are fine, bad or missing, reverse DNS can land your outreach in spam.

For outreach teams, misaligned infrastructure creates a mismatch between what your email says it is and what the server records prove. Gmail and Outlook use those mismatches as a reason to send emails to spam. The risk goes up if you’re adding new inboxes or domains without proper setup, your emails look unverified, even if your copy and targeting are solid.

How to fix it:

Make sure your sending IP has a PTR record that resolves back to your domain, and that SPF, DKIM, and DMARC are aligned with the same domain identity. 

Stick with Google/Microsoft’s own infrastructure or a reliable shared pool, don’t rush into dedicated IPs unless you’re sending 100k+ emails daily, a highly unlikely scenario in B2B outreach. 

Spammy content (and how to clean it up)

Even with perfect authentication, content can still tank deliverability. Gmail has gotten very good at spotting emails that look like spam: subject lines in ALL CAPS, fake reply threads, and exaggerated claims. Formatting mistakes, like sending one big image or skipping a clear unsubscribe path, trigger filters too.

The fix is simple but strict: write like a real person. Keep subject lines short and honest, avoid manipulative tricks, and use clean, plain-text style formatting. Before you load a sequence, test it.

Faulty prospect data (and how to clean it)

Fake email IDs are another hidden killer, one that affects you deeply since you’ll be reaching out to lots of cold prospects who have not engaged with you before. High bounce rates, spam traps, or emailing unresponsive contacts tells Gmail and Outlook you’ve scraped lists and that you’re probably emailing people who may not want to hear from you. That tanks your reputation even if your copy and setup are flawless.

How to fix it:

Go for ruthless list hygiene. Verify emails before sending, suppress bounces immediately, and cut prospects who never engage. Buying scraped data is the fastest way to burn domains; it almost always contains traps that will crush your deliverability. Smaller, engaged lists send better signals and keep your domains alive.

Sudden sending spikes (and how to scale without burning)

Mailbox providers hate surprises. If you’ve been sending 200 emails/day and suddenly ramp to 2,000, Gmail assumes it’s a spammer move, even if the list is clean resulting in throttling, delays, or mass spam placement.

How to fix it:

Scale slowly to remedy this. Warm new domains before you use them for live campaigns, and raise daily caps in small increments. 

Use MailReach’s warm-up to send safe, realistic traffic in the background so Gmail sees history building before you scale. 

Monitoring and feedback loops (and how to stay ahead)

Even after you fix issues, filters change daily. That’s why monitoring matters. Google Postmaster Tools tells you your domain’s reputation and complaint rate. Microsoft SNDS shows how Outlook views your IP traffic. If either dips, it’s a signal to slow down and increase warm-up until the domain reputation recovers.

Bringing It All Together: Your Path to the Inbox

Email deliverability issues can escalate quickly. With a structured approach, you can proactively prevent most problems and address any issues efficiently when they arise.

Here’s the sequence to remember:

1. Get the technical basics right with authentication, alignment, and consistent sending.
2. Build and protect your sender reputation by warming up domains, avoiding spam reports, and focusing on engagement.
3. Write clean, relevant emails and send them to a healthy, active list.
4. Monitor continuously so you spot problems early and stay off blacklists.

When your domain is authenticated, your content is clear, and your recipients engage, mailbox providers reward you with high deliverability. And that’s where your emails can do what they’re meant to: reach people, spark action, and drive results.

For email warm-up built for your outreach, try MailReach now.