Why the best email in the world is worthless in a spam folder -- and how to make sure yours lands in the inbox.
Rees Bayba
Founder, Astra GTM
TL;DR
Email deliverability is the ability of an email to successfully reach the recipient's inbox rather than their spam folder or getting rejected outright. It is not just about whether the email was sent -- it is about where it lands. An email can be sent, received by the mail server, and still never be seen by the person it was intended for. Deliverability is determined by four factors: authentication, sender reputation, content quality, and list quality.
Copy is the ceiling of your campaign. Deliverability is the floor. If your email lands in spam, the copy is irrelevant -- the recipient never sees it. A mediocre email that reaches the inbox will always outperform a brilliant email in the junk folder. Before spending time refining personalization or subject lines, confirm that your emails are actually landing in inboxes. Most deliverability problems are silent: sends look successful, open rates plummet, and the cause is invisible until you run a diagnostic.
Authentication proves you are who you say you are. Without it, receiving mail servers have no way to verify your emails come from a legitimate sender, and most will route them to spam. All three records must be configured correctly before sending any cold email.
Sender reputation is a score that mail providers assign to your sending domain and IP address based on past sending behavior. It is built through consistent, engaged sending over time. High spam complaints, high bounce rates, and sudden volume spikes all damage it. New domains start with no reputation -- which is why warmup is mandatory before any cold email campaign.
The content of your emails is processed by spam filters before delivery. Spam filters flag patterns associated with mass commercial email: heavy HTML formatting, image-heavy layouts, excessive links, spam trigger words, and text that does not look like it was written by a person.
Your list is a reflection of your deliverability care. Sending to invalid addresses generates hard bounces. Sending to disengaged contacts generates soft signals that lower your reputation. Sending to spam traps -- addresses maintained by blacklist operators specifically to catch careless senders -- can get your domain blacklisted immediately.
Gmail and Outlook both use proprietary filtering systems, but the inputs are knowable. Understanding the decision logic tells you which levers to pull when deliverability drops.
Outlook is the hardest major inbox provider to land in. Its SmartScreen filter and proprietary reputation databases are less transparent than Gmail. If your ICP is heavy on corporate Microsoft users, deliverability must be a primary strategy, not an afterthought.
Not all email metrics are created equal. Some have been made unreliable by privacy changes. Focus on the metrics that reflect real human behavior.
| Metric | What it indicates | How to improve |
|---|---|---|
| Spam rate | % of delivered emails marked as spam. Keep under 0.1% for Gmail (hard cap). Above 0.3% triggers filtering. | Better list quality, more relevant targeting, easier unsubscribe, stop sending to disengaged contacts. |
| Bounce rate | % of sends that resulted in a hard bounce (invalid address). Keep under 2%. | Email verification before every send. Never use purchased lists. Re-verify lists older than 90 days. |
| Reply rate | The most honest deliverability signal. If emails are reaching inboxes and resonating, people reply. | Improve copy, tighten ICP, reduce send volume to increase targeting quality. |
| Open rate | Unreliable since Apple Mail Privacy Protection (Sept 2021). Machine opens inflate the metric. Do not use as a primary indicator. | N/A -- measure reply rate instead. |
| Domain reputation | Google Postmaster Tools rates your domain: High, Medium, Low, Bad. Low = some filtering. Bad = near-universal spam folder. | Reduce sending volume, improve list quality, increase engagement with existing contacts. |
| Blacklist status | Whether your sending domain or IP appears on major blacklists (Spamhaus, Barracuda, etc.). | Use MXToolbox to check. Delist by addressing the root cause (bounces, spam complaints) and submitting a removal request. |
Microsoft Outlook and Office 365 use a filtering system called SmartScreen, combined with their own domain and IP reputation databases that are separate from Gmail's. In Q1 2025, average Outlook inbox placement for cold email dropped to 26.77% -- meaning nearly three-quarters of cold emails to Outlook recipients are going to spam or junk. The problem is compounded by a lack of diagnostic tools: there is no free equivalent to Google Postmaster Tools for Microsoft.
What is a good email deliverability rate?
Inbox placement rate above 85% is considered acceptable. Above 90% is good. For Gmail specifically, domain reputation of 'High' in Google Postmaster Tools corresponds to strong inbox placement. The more actionable metric is keeping spam rate under 0.1% -- that is the threshold above which Gmail begins throttling delivery.
How do I check my email deliverability?
Start with Google Postmaster Tools -- register your sending domain and monitor spam rate and domain reputation weekly. Run MXToolbox blacklist checks monthly. For a point-in-time test, send to a Mail-Tester address and review the detailed report. If you use a cold email platform, check whether it shows inbox placement data.
Why is Google easier to land in than Outlook?
Gmail has more transparent reputation signals (Google Postmaster Tools shows your domain reputation directly), uses engagement as a positive signal (replies improve your standing), and has more consistent filtering behavior. Microsoft's SmartScreen is less transparent, maintains its own proprietary databases, and tends to be more aggressively conservative -- particularly for new domains and cold outreach patterns.
How long does it take to fix a deliverability problem?
It depends on the cause. Authentication issues (SPF, DKIM, DMARC) can be fixed in hours. Recovering from a blacklisting takes 2-4 weeks after the root cause is addressed and a removal request is submitted. Rebuilding domain reputation from Low or Bad to High takes 4-8 weeks of clean, low-volume sending with strong engagement signals. There is no shortcut -- the only path is consistent clean sending over time.
Does domain age affect deliverability?
Yes. New domains (under 30 days) have no reputation history, so mail providers apply conservative filtering by default. Warmup -- gradually increasing volume from a new domain -- signals consistent, legitimate sending behavior. Never start cold outreach from a domain purchased last week. Register domains at least 30 days before you intend to use them, and warmup for a minimum of 4 weeks before sending at scale.
Does email warmup actually help?
Yes, but it is a baseline, not a fix-all. Warmup builds initial domain reputation by simulating engaged sending behavior -- warmup tools send emails between their network of mailboxes and automatically open/reply to them. This signals to mail providers that the domain sends legitimate, engaged-with mail. Without warmup, cold outreach from a new domain will go to spam for the first several weeks. With warmup, inbox placement is achievable from the start. Warmup does not compensate for bad list quality or poor authentication.
We implement these systems end-to-end. First sends within 14 days.