You improve email deliverability by proving your mail is really yours (SPF, DKIM and DMARC), sending from sound infrastructure, mailing only people who want your messages, and reading the diagnostics mailbox providers give you. No setting guarantees inbox placement, because Gmail, Microsoft, Yahoo and others make their own filtering decisions. What you can do is remove every avoidable reason for rejection or spam placement.
This guide suits anyone sending from their own domain: a groomer’s appointment reminders, a rescue’s adoption newsletter, a vet clinic’s recall notices, or a pet-supply shop’s order emails. Provider rules differ, so each requirement below names who it applies to.
How do I improve email deliverability?
Work through five layers in order. Each one fixes a different kind of failure:
- Identify every system that sends mail using your domain.
- Authenticate that mail with SPF, DKIM and DMARC, and confirm alignment.
- Check the sending infrastructure and message format.
- Protect recipient trust: opt-in, relevant content, easy unsubscribe, clean lists.
- Diagnose by provider, using Postmaster Tools and the SMTP error codes in your bounces.
Why are my emails going to spam or bouncing?
The usual causes map to the layers above:
- Failed or missing authentication. Google states that messages not authenticated with the required methods might be marked as spam or rejected with a 5.7.26 error.
- A forgotten sender. A newsletter tool or booking system that sends as your domain but is not in your SPF record or signing with DKIM. Google warns that omitted third-party senders can be more likely to be marked as spam.
- High complaint rates. Recipients who did not expect or want the mail click “report spam.”
- Infrastructure problems. Missing reverse DNS, no TLS, or malformed message structure.
- Retrying dead addresses. Continuing to send to recipients who have permanently refused your mail damages your standing.
Step 1: Inventory every source that sends as your domain
List everything that sends mail with your domain in the From address:
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- Marketing or newsletter platform
- Transactional mail (order confirmations, password resets)
- Booking, scheduling or practice-management software
- CRM and support desk
- Website contact and donation forms
- Your regular mailbox provider
Each one must be covered in your authorized SPF configuration and authenticated, ideally with DKIM signed using your own domain. Skipping the inventory is the most common reason a setup that “looks done” still fails for one category of mail.
Step 2: Set up SPF, DKIM and DMARC and verify alignment
What each mechanism does
| Mechanism | What it checks | Limitation |
|---|---|---|
| SPF | Uses DNS to list sources authorized to send for the MAIL FROM domain | Does not by itself validate the visible From domain |
| DKIM | Lets receivers validate a cryptographic signature tied to a signing domain | The signing domain may differ from the visible From domain |
| DMARC | Checks alignment between SPF/DKIM results and the visible From domain, sets the policy for failures, and can direct reports to you | Only passes if at least one aligned mechanism passes |
Microsoft’s authentication documentation (Microsoft Learn, “How email authentication works in Microsoft 365”) puts it plainly: internet domains need SPF, DKIM and DMARC to work together. It also notes that forwarding or message modification can disrupt authentication, so a failure on forwarded mail does not always mean your setup is wrong.
Gmail’s requirements, by sender size
These apply to mail sent to personal Gmail accounts. Google’s sender requirements took effect on February 1, 2024, and are not universal rules for every mailbox provider.
| Sender | Requirement |
|---|---|
| Any sender to personal Gmail accounts | Set up SPF or DKIM; Google recommends all three (SPF, DKIM, DMARC) |
| More than 5,000 messages per day to Gmail accounts | SPF, DKIM and a DMARC record (Google says the policy may be p=none); the visible From domain must align with SPF or DKIM for direct mail |
| Marketing and subscribed mail (bulk senders) | One-click unsubscribe plus a visible unsubscribe link in the body |
A small pet business will rarely approach 5,000 Gmail messages per day, but the lower tier still applies, and aligned authentication is the cheapest protection you can add. Start DMARC at p=none with reporting on, confirm all legitimate sources pass, and only then consider a stricter policy.
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Google’s FAQ says user-reported spam rates should stay below 0.1% and should never reach 0.3% or higher. At 0.3% you lose eligibility for mitigation, and you regain it once the reported rate stays below 0.3% for seven consecutive days. These are Google’s operational thresholds for Gmail, not guarantees of inbox placement anywhere.
Step 3: Check sending infrastructure and message format
- Forward and reverse DNS. Sending IPs should have valid forward and reverse (PTR) records. Microsoft also calls for valid reverse DNS and active management of outbound IP reputation.
- TLS. Use it for transmission; it is part of Gmail’s requirements.
- RFC 5322 formatting. Headers and structure must be standards-compliant.
Gaps here can produce temporary or permanent failures, spam-folder placement, or loss of mitigation eligibility. If you send through a reputable email service provider (ESP), most of this is handled for you, but confirm that the provider authenticates your own domain rather than only its shared one.
Step 4: Protect recipient trust and list quality
Authentication proves who you are. Reputation reflects how recipients treat your mail over time.
- Get explicit opt-in. Google recommends it. Adoption-event attendees or customers who bought a leash did not necessarily ask for a weekly newsletter.
- Send relevant, expected content at a reasonable volume. If someone signed up for vaccination reminders, do not send them a promotional blast.
- Make unsubscribing easy. Gmail requires one-click unsubscribe for applicable marketing and subscribed mail, and unsubscribe requests must be honored within 48 hours.
- Stop retrying permanent failures. Microsoft’s sender rules say that after a 5xx permanent non-delivery response, the sender must not attempt to retransmit that message to that recipient, and should stop after multiple non-delivery responses. Remove those addresses from your list.
Step 5: Diagnose by provider and signal
Gmail: Postmaster Tools
Google Postmaster Tools reports compliance status, spam rate, IP and domain reputation, SPF/DKIM/DMARC authentication, encryption and delivery errors for mail to personal Gmail accounts. Two caveats: the data is not real time, and Google notes compliance changes can take up to seven days to appear. Low sending volume may also mean some dashboards show no data, which is not itself a problem.
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Microsoft-hosted recipients
Read the SMTP error codes in bounce messages and the authentication results in received headers. Microsoft’s guidance for bulk senders sending on behalf of another domain is to verify that authentication aligns with that domain.
Yahoo
Yahoo’s Sender Hub guidance emphasizes sender/IP and DKIM-domain reputation and recommends separating bulk marketing traffic from user, transactional and alert mail where appropriate. Check Yahoo’s current published requirements directly before relying on specifics, as they were not confirmed in detail here.
Quick troubleshooting map
| Symptom | Likely cause | First check |
|---|---|---|
| Rejections with 5.7.26 at Gmail | Authentication failure | SPF/DKIM/DMARC pass and alignment in Postmaster Tools and message headers |
| One tool’s mail lands in spam, others fine | That source missing from SPF or not DKIM-signing your domain | Your source inventory |
| Gradual drop in placement | Rising complaints, stale list | Spam rate against the 0.1% and 0.3% thresholds |
| Repeated 5xx bounces | Invalid or refusing recipients | Suppress those addresses |
| Fails only on forwarded mail | Forwarding or modification breaking authentication | Whether DKIM survives and aligns |
Choosing an ESP or monitoring tool
Google advises verifying that third-party providers follow its sender rules and authenticate your domain. When comparing ESPs, look at:
- Custom-domain SPF/DKIM support and DMARC alignment
- Separate handling of transactional and marketing mail
- Bounce and complaint handling
- Sending-identity separation
- Provider-specific reporting
- Integration fit and price at your expected volume
For DMARC reporting or deliverability monitoring, start with the free mailbox-provider dashboards. Postmaster Tools is Gmail-specific and not real time, so a paid monitor is worth considering only if you run several domains or sending services and need to see them in one place.
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What no one can promise
The figures above are provider policy and operating thresholds, not industry statistics, and no source reviewed supports a promised inbox-placement rate. Requirements change, so recheck Google’s, Microsoft’s and Yahoo’s current sender documentation periodically.
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