Email deliverability 101: What actually goes into a healthy email program

TL;DR

Email deliverability is a foundational concern for customer engagement marketers—brands that neglect this key area of focus can see their messages delayed or even blocked, making it hard to see the full value of this impactful channel. By embracing email authentication, IP warming, and other key deliverability best practices, marketers can bolster their sender reputation and avoid potential penalization by inbox providers that could undermine the effectiveness of their email programs.

Key takeaways

• Brand that send high volumes of emails on unwarmed IPs run the risk of adverse consequences from inbox providers
• Using purchased or borrowed email lists can compromise email list hygiene and undermine the overall effectiveness of an email marketing strategy
• Neglecting email authentication can trigger issues with Gmail, Yahoo!, and other prominent inbox providers

Email deliverability is not just about sending the right message to the right person at the right time—it's also about whether that message reaches the inbox. Every email you send is evaluated by mailbox providers, who determine whether it belongs in the inbox, the spam folder, or should be blocked entirely. This guide breaks down what healthy email deliverability really means beyond hitting the send button. You learn about the key factors mailbox providers evaluate, what positive signals to look for, and what negative signals can quietly undermine your program.

List acquisition

At the foundation of every great email program is list acquisition: how you collect email addresses. The source and quality of your list will impact your sending practices, especially if data quality is poor. Mailbox providers can detect bad sending behaviour, and it starts at the beginning of your email sending journey.

Good list acquisition means using a clear sign-up form with explicit opt-in consent and communicating how frequently subscribers can expect to hear from you. This should be an opt-in where subscribers sign up with you directly, not through a third party. To ensure you're capturing quality addresses, use double opt-in and watch for typos, bots, role-based addresses, and low-intent sign-ups.

Poor acquisition practices are where most programs run into trouble before they hit send. Rented, scraped or purchased lists should never be used and are against many reputable ESP’s Acceptable Use Policies. The best practice is to avoid using pre-checked opt-in boxes and importing old lists that haven't been mailed in over 12 months. Clear, explicit consent, as well as expectations around communication frequency and message content, should be communicated at the point contacts subscribe to your mailing list.

Authentication

This is also why authentication matters. Without it, mailbox providers have no way to verify that your emails are legitimate, and will treat them as suspicious or route them directly to the spam folder. Authentication is made up of three DNS records, SPF, DKIM, and DMARC, and you need all three in place before you can send.

SPF

SPF, or Sender Policy Framework, is a DNS record on your sending domain that lists every IP address authorized to send email on your behalf. When a mailbox provider receives your email, it checks whether the sending IP is on that list. If it is, SPF passes. If not, SPF fails and the email may be flagged as unauthorized or spoofed. Every IP you send from must be included in your SPF record. When Braze sets up your IPs and domains your SPF record is also configured.

DKIM

DKIM, or DomainKeys Identified Mail, adds a cryptographic signature to every outbound email using a public key published in your DNS. This verifies that the message came from your domain and wasn't altered in transit. DKIM is the record mailbox providers use to tie your sending behavior to your domain's reputation over time. To break it down:

• Your SPF verifies the sending IP
• Your DKIM verifies the message itself and builds the reputation history that follows your domain

If DKIM isn't set up, you aren't building a reputation, you're starting from zero every time. When you get started with Braze we set up your DKIM records when your IPs and domains are configured. The only step you need to take is adding the DNS records we provide.

DMARC

DMARC, or Domain-based Message Authentication, Reporting, and Conformance, tells mailbox providers what to do when SPF or DKIM fail.

Your policy options are:

• Monitor (p=none)
• Send to spam (p=quarantine)
• Block entirely (p=reject).

We recommend confirming all legitimate sending sources are authenticated, then work toward p=reject, the standard that signals your domain is protected. Without DMARC, your domain can be spoofed; email sent in your name damages your reputation even though you never sent it.

Before your first send, you need:

1. Your SPF record published and inclusive of all sending IPs.
2. DKIM keys configured on every sending stream.
3. Your DMARC record published at minimum p=none with a reporting address you actively monitor.

While SPF and DKIM are configured by Braze, DMARC is set up by the sender. You will need to publish a DMARC record to your DNS to be fully compliant with mailbox provider requirements.

Sending domains

Sending from a subdomain rather than your root domain helps contain reputation signals and protects your main domain if something goes wrong.

Marketing emails should always be sent from a subdomain of your root domain. If a spam complaint, sending spike, or list quality issue occurs, the damage is contained to the subdomain and your root domain stays clean.

In Braze, your sending IP is a dedicated IP, meaning you own its reputation entirely. Your sending behavior builds that reputation, good practices strengthen it, and mistakes damage it. All IPs must be warmed before sending to full volume.

Transactional and marketing emails should always be separated onto different IPs, since transactional email naturally carries high engagement and low complaints. Mixing the two risks dragging down your reputation and causing critical transactional emails to go undelivered.

IP warming

IP warming is necessary because brand new IPs have a neutral reputation. Sending a large volume from an unwarmed IP will cause most mailbox providers to treat it as suspicious, and you risk temporary or permanent blocks before you've had a chance to establish yourself. IP warming is the process of gradually increasing sending volume over several weeks, starting with your most engaged subscribers and steadily increasing volume as positive signals also increase.

Sending to a highly engaged audience signals to mailbox providers that recipients are responding positively. High open rates, low complaint rates, and absence of spam traps all help to build reputation and ensure high performance.

Braze offers three IP warming schedules as a guideline for your IP warming strategy as well as an IP warming guide packed with tips and insight for your IP warming journey. The conservative schedule is designed for senders who are brand new to email, migrating from a shared IP, or who have experienced deliverability issues such as throttling or blocklisting. Many senders apply the moderate schedule. The aggressive schedule is appropriate for senders with strong, well-established practices, but it does come with naturally increased risk. Braze strongly advises against skipping days during warming; consistent daily scaling is what builds trust with mailbox providers.

No matter which IP warming schedule is best for your email program considerations and goals, monitoring is essential throughout. If you see complaints rising or throttling occur, investigate the cause, fix it, and hold steady to evaluate the remedy before continuing to ramp. IPs can carry reputation signals for approximately 90+ days, after three months of non-use.

Sending patterns

One of the most common mistakes senders make is an inconsistent sending cadence. Mailbox providers look for steady, predictable sending patterns.Sudden volume spikes raise flags. If you know a large campaign or seasonal send period is coming, ramp up gradually ahead of it the same way you would during IP warming. If you've been inactive for a period, treat your restart like a mini IP warmup. For sends with unusually high volume that are outside of regular sending patterns, rate limiting is a good option, especially for smaller or regional mailbox providers that can't absorb large spikes, allowing mail to be delivered at a controlled rate rather than all at once.

Monitoring

Monitoring your email program's health begins with postmaster tools, available from providers like Google Postmaster and Microsoft SNDS, both of which are accessible via the Braze Deliverability Center. These tools give you visibility into how your mail is being evaluated: where your IP reputation stands, and whether you're seeing spam complaints. If you're not checking this information, you're missing clear signals about the health of your email marketing program.

Your recipients can also play a role in the health of your email program. Spam complaints, which happen when your email is marked as spam by a recipient, can impact your email deliverability. To help senders manage complaints, many mailbox providers use feedback loops (FBLs) which are automated systems that report back what a user flags and email as junk though the depth of data shared varies depending on the provider’s privacy policy. If you are a Braze customer, we sign up for and manage FBLs automatically on your behalf.

Delivery Rate and Email Reputation

Delivery rate is the percentage of sent emails that were accepted by the receiving mail server, meaning they were not bounced. A delivery rate of 99% and a bounce rate under 1% are the benchmarks to aim for, but hitting those numbers doesn't mean your program is healthy. Delivery rate measures acceptance by the server, not inbox placement. so an email can be "delivered" and still end up in the spam folder.

The two mail components of email reputation are IP reputation, which is tied to the specific IP address you send from, and domain reputation which is tied to your subdomain.

• Google Postmaster shows your domain and IP reputation using low, medium and high
• Microsoft SNDS uses Red, Yellow and Green as metrics for signals for reputation reflecting the past 30 days of sending activity.

Email reputation is a set of signals that mailbox providers use to evaluate your sending domain and IP address based on your historical sending behaviour. Each mailbox provider, including Yahoo and iCloud have their standards for best practices, so it is vital you stay on top of what each mailbox provider’s requirements are.

Positive Email Program Signals

Mailbox providers evaluate how recipients respond to your emails and use those signals to inform inbox placement decisions. Positive engagement signals indicate to mailbox providers that recipients are engaged and they want to receive messages from you. Opens, forwarding, and replies are all positive engagement signals that mail providers look for and show that your subscribers are interested in and expecting your content. The primary engagement metrics mailbox providers look for include:

• Open Rate: The total number of email opens your messages receive, divided by the number of emails delivered.
• Click Rate: The total number of clicks your emails receive, divided by the total number of emails delivered.
• Unique Open Rate: The total number of unique recipient email opens divided by the number of emails delivered in the campaign.

Negative Email Signals

The other side of engagement metrics are negative signals which put your messages at risk of being rejected or marked as spam by mailbox providers.These signals can cause damage to your reputation. Emails that are not opened, are deleted without being read, or are reported as spam, even by recipients who may have previously opted in, can harm your email program by signaling to mailbox providers that your messages are unwanted or unexpected.

• Spam Complaint Rate: The number of email recipients who report your emails as spam divided by the number of emails you delivered. Spam complaints are a direct signal from recipients to mailbox providers that the mail received is unwanted and is taken seriously by mailbox providers as high complaint rates can cause a decline in reputation. Keep these rates low to stay in compliance with mailbox provider best practices.
• Bounce Rate: Hard bounce rate is the number of emails that weren’t delivered due to invalid or nonexistent email addresses divided by the total number of emails sent.
• Hard bounces: These are permanent failures, triggered when the address doesn't exist, the domain is invalid, or the receiving server has permanently rejected you. Soft bounces are temporary failures, delivery couldn't complete due to a transient issue, such as mailbox full or a typo in the domain.

How to practice good list hygiene

The final layer of email deliverability—one that is often overlooked—is list hygiene. Your delivery rate and reputation are all impacted by this critical aspect of your email program. List hygiene is the practice of keeping your list accurate, current, and full of subscribers who want to hear from you and are engaging with your content.

List hygiene is not a one-time task; it's a continuous process. Typically, we recommend that list cleaning should take place a minimum of every six months. Here are several practices that help to keep your data clean over time:

• Bounce management: Braze automatically suppresses hard bounces. Senders can temporarily suppress soft bounces to catch addresses that are consistently failing with the soft bounce filter. Examples of common soft bounces that can be temporarily suppressed are Mailbox Full: 452-4.2.2 The recipient's inbox is out of storage space (Gmail). The soft bounce suppression list contains groups of users who automatically do not receive any campaigns or canvases based on the segment filter and criteria that you put in place. Users will enter and exit suppression lists as they meet filter criteria (e.g. two soft bounces in the past seven days).
• Engagement-based suppression: This means removing or suppressing contacts who haven't opened, clicked, or interacted with your emails over a defined window (commonly, 30, 60, or 90 days). You can do this with a Sunset policy, creating a rule that says after “x” months of inactivity and a failed re-engagement attempt, a contact is permanently suppressed. You can also create suppression lists for typo or invalid emails using a regex filter. One of the most common types of bounces for invalid emails or domains is, for example: 550-5.1.1 The email account that you tried to reach does not exist. Please try\r\n550-5.1.1 double-checking the recipient's email address for typos (Gmail)
• Re-engagement campaigns: Before suppressing inactive contacts, you can send a targeted campaign to these recipients that specifically asks them if they are still interested in hearing from your brand.
• List validation: Running your list through a verification service that checks whether addresses are real, formatted correctly, and not role-based or disposable can help to support improved list hygiene.

Final thoughts

Deliverability is ultimately a business-critical metric that determines whether your emails reach the inbox and achieve their intended impact. When messages land in the spam folder, they fail to reach customers who actively want to hear from you, providing no meaningful value to the recipient and no return on investment for the sender. Poor deliverability can also have broader consequences beyond marketing campaigns. A damaged sending reputation can affect your entire customer base, including users who rely on important transactional emails such as receipts, password resets, and account notifications. When trust with mailbox providers declines, even essential communications can be disrupted.

For this reason, deliverability should not be treated as a one-time configuration or a set-it-and-forget-it task. It is an ongoing discipline that requires consistent monitoring, maintenance, and optimization. The investment you make in protecting sender reputation directly determines whether your messages reach the inbox, engage your audience, and deliver business value at scale.

Interested in diving deeper into deliverability? Check out our guide to IP warming and our look at email authentication.