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Before you spend a week trying to improve deliverability, spend an hour measuring it. These five tests give you a baseline across authentication, infrastructure, reputation, and actual inbox placement, using mostly free tools.
Run them in order. Each builds on the previous one.
Test 1: Authentication Check (5 minutes)
The first question is whether your mail can even prove it's yours. Use the free domain health tool and check:
- SPF: present, syntactically valid, under the 10-DNS-lookup limit.
- DKIM: selector resolves, key is 1024-bit or larger.
- DMARC: record exists, includes a rua reporting address.
- MX: points to your actual mail server, no surprises.
Run this for every sending domain, including any subdomain your ESP uses. Issues at this layer cause the most deliverability damage.
Test 2: Mail-Tester Score (10 minutes)
Mail-tester.com gives you a free, unique email address. Send a real campaign to it (use the actual template you'd send to customers) and check the score.
What to look at:
- Overall score: aim for 9.5+/10.
- SpamAssassin breakdown: any rule scoring >0.5 needs investigation.
- Authentication section: all three checks should pass.
- Content section: flags like missing unsubscribe, image-only emails, suspicious links.
The free MailVitals email deliverability test works the same way: send one real message to your own test address and get a report scored out of 10. To test the copy alone, the email spam checker runs Apache SpamAssassin's published content rules.
Test 3: Seedlist Inbox Placement (30 minutes)
Mail-tester scores tell you what should happen. Seedlist tests tell you what actually happens at Gmail, Outlook, Yahoo, etc.
A seedlist test sends your campaign to dozens of test inboxes across major providers, then reports where it landed: inbox, promotions, spam, or missing. Tools that do this include GlockApps and Litmus.
Target rates:
- Gmail inbox: >85%
- Outlook inbox: >80%
- Yahoo inbox: >85%
- Anywhere missing/blocked: <5%
Anything below these targets means a specific provider has a reputation issue with your domain, and you should dig into that provider's postmaster tools.
Test 4: Reputation Check (Postmaster Tools)
For each Gmail-active sending domain, set up Google Postmaster Tools (free). Wait a few days for data, then check:
- Domain reputation: should be "High" or "Medium". "Low" means immediate action; "Bad" means you're being throttled or rejected.
- IP reputation: same scale.
- Spam rate: under 0.1%. Above 0.3%, Gmail's published sender rules take away delivery mitigations and spam placement climbs.
- Authentication pass rate: should be 99%+.
- Encryption rate: TLS coverage; should be near 100%.
For Microsoft, sign up for SNDS to get the equivalent IP-level data.
Test 5: Blacklist Check (5 minutes)
Run your sending IPs and domain through a blacklist scanner. The free blacklist checker covers Barracuda, SpamCop, UCEPROTECT and the rest of the 25 public lists. Spamhaus refuses lookups that come through public DNS resolvers, so check it on Spamhaus's own lookup page.
What to do with results:
- Listed on Spamhaus ZEN: highest priority. Contact Spamhaus immediately with evidence of remediation.
- Listed on Barracuda or SpamCop: moderate impact. Follow the delisting process on the listing's page.
- Listed on UCEPROTECT-2 or -3: low impact (these are escalation lists with high false positive rates).
For ongoing monitoring across 25 DNSBLs, automated tools alert you within hours.
Putting It Together: Your Deliverability Health Card
After all five tests, you'll have a complete picture. Score yourself:
| Test | Pass | Warning | Fail |
|---|---|---|---|
| Authentication | All three green + aligned | One warning | Any failure |
| Mail-tester | 9.5+/10 | 8.5-9.4 | <8.5 |
| Inbox placement | >85% across providers | 70-85% | <70% |
| Postmaster Tools | High reputation | Medium | Low/Bad |
| Blacklists | Clean | Listed on UCEPROTECT only | Listed on Spamhaus/Barracuda |
Three or more "Pass" rows: you're in good shape, so work on the warnings. Any "Fail" row: stop sending bulk mail and remediate first. See the improvement playbook.
How Often to Re-Test
- Authentication: monitor continuously. DNS edits can break it without warning.
- Mail-tester: every new template, every major content change.
- Seedlist: weekly during a deliverability project, monthly when stable.
- Postmaster Tools: weekly.
- Blacklists: daily ideally, weekly minimum.
FAQ
What's the difference between deliverability and delivery rate? Delivery rate is the percentage of messages accepted by the receiving server (vs hard bounced). Deliverability (or inbox placement) is the percentage that landed in the inbox vs spam/missing. You can have 99% delivery and 30% inbox placement at the same time.
Why does mail-tester say I'm fine but seedlist tests show poor inbox placement? Mail-tester checks content and authentication mechanics. Seedlist tests reflect actual sender reputation at major providers. Reputation is invisible to mail-tester.
How long do reputation problems take to show up? Postmaster Tools reflect reputation with a 1-3 day lag. Real-world inbox placement degrades within hours of a reputation hit.
Do these tests work for transactional email? Yes. Run mail-tester against a real password reset, run authentication against the transactional sending domain, and check Postmaster Tools for the same domain.
Bottom Line
If you can't answer "what's my current inbox placement rate at Gmail?" you can't improve it. These five tests take under an hour total and give you the baseline you need.
👉 Continue: How to Improve Email Deliverability · What is a Good Email Deliverability Rate? · How Do I Know If I'm Blacklisted?
Sources
- Email sender guidelines (Gmail Help), the authentication, DKIM key length (1024 bits minimum) and spam-rate requirements Gmail sets.
- Email sender guidelines FAQ (Gmail Help), the 0.1% and 0.3% spam-rate thresholds and what each costs a bulk sender.
- Set up Postmaster Tools (Gmail Help), setting up Postmaster Tools for each sending domain.
- Smart Network Data Services FAQ (Outlook.com), what SNDS reports per IP and how its colours are defined.
- ZEN Blocklist (Spamhaus), what Spamhaus ZEN combines (SBL, CSS, XBL, PBL).