Overview
Getting your emails into the inbox isn't just about technical setup — it's about
who you're sending to and how they respond. Mailbox providers (MBPs) like Google and Microsoft use recipient engagement as a key signal when deciding where your mail lands. This guide walks you through the most important steps you can take to protect and improve your deliverability.
1. Start With a Quality List
Your list is the single biggest factor in your deliverability. When recipients don't want your mail —ignoring it, deleting it, unsubscribing, or marking it as spam — those signals accumulate and damage your sender reputation over time. On the flip side, an engaged list actively helps your reputation.
What to do:
Only send to recipients who explicitly asked to receive your emails. Never use scraped, purchased, or third-party lists — these almost always lead to higher complaint rates and reputation damage.
On your signup forms, make sure the recipient completes the opt-in action themselves. Avoid pre-checking the opt-in box, as this does not constitute genuine consent.
2. Keep Your List Clean — Continuously
List hygiene isn't a one-time task. Sending to invalid addresses or users who have previously complained sends a strong negative signal to MBPs.
Practice | Recommendation |
Bounce handling | Automate and apply consistently |
Complaint processing | Automate and apply consistently |
Removing unengaged users | Every 90–120 days |
Set up a sunset policy to stop mailing recipients who haven't engaged within 90–120 days. Keeping unengaged users on your list hurts the deliverability of mail going to your engaged recipients too.
3. Engagement Is Everything
Providers like Google and Microsoft closely monitor whether your recipients open, click, and interact with your mail. If a large portion of your sends are going to unengaged users, it can drag down inbox placement across your entire program — even for your most engaged subscribers.
Send only to people who want and expect your messages, and make sure your sending frequency aligns with their expectations.
4. Make It Easy to Unsubscribe
If recipients can't easily opt out, they'll mark your email as spam instead — and complaint activity is one of the most damaging signals you can generate with MBPs.
Include a clear, visible unsubscribe link in every message.
Include list-unsubscribe headers — many MBPs, particularly Outlook/Hotmail, will filter mail that's missing them.
5. Separate Your Mail Streams
If you're sending both promotional and transactional emails from the same domain or IP, lower engagement on your marketing traffic can affect the deliverability of your more critical transactional messages (like password resets or receipts).
We recommend separating mail streams by subdomain or IP to keep your reputations isolated from one another.
6. Monitor Your Reputation Regularly
Don't wait for a deliverability problem to appear — check in on your reputation proactively using these free tools:
Google Postmaster Tools — view your domain and IP reputation, complaint rates, and delivery trends at Gmail.
Microsoft SNDS— monitor your IP reputation and complaint data at Outlook/Hotmail.
7. Content Best Practices
Format Your Emails Correctly
Always send multi-part emails (HTML + plain text). HTML-only emails are not well received by MBPs, and since many block images by default, your email may render poorly for a large portion of recipients.
Keep your text-to-link and text-to-image ratios high — too many links or images can trigger spam filters.
Avoid ALL CAPS, excessive exclamation marks!!!, and spammy phrases
like "Buy Now!" or "Free!" — these are common spam flags.
Proofread carefully — misspellings are also flagged by spam filters.
Keep Your Domains Consistent
The domains in your From field, Return-Path, and Message-ID should all match your sending domain.
Message IDs must be correctly formatted with angle brackets <> and the correct domain after @. Gmail pays close attention to this.
Use your sending domain in your links. Avoid popular URL shorteners, as they are frequently associated with spam and can trigger filters.
Personalize Your Emails
At a minimum, address each recipient by name.
Where possible, tailor content to reflect the recipient's specific interests or behavior in your application.
Use Paytronix parameters with your email templates to personalize at scale without extra effort.
Write Effective Subject Lines
Your subject line should accurately reflect the content of the email. Misleading or unexpected subject lines increase complaint risk and reduce engagement.
A/B test your subject lines — they have a significant impact on open rates.
Deliverability Checklist
All recipients have explicitly opted in — no purchased or scraped lists
Opt-in box is not pre-checked on signup forms
Bounce and complaint handling is automated
Sunset policy in place for unengaged users (90–120 days)
Every email includes a visible unsubscribe link and list-unsubscribe headers
Promotional and transactional mail are on separate streams
Google Postmaster Tools and Microsoft SNDS are set up and reviewed regularly
Emails are sent as multi-part (HTML + plain text)
Links use your sending domain — no URL shorteners
Message-ID is generated
From, Return-Path, and Message-ID domains all match
Emails are personalized using recipient variables
Subject lines are A/B tested regularly