Deliverability mechanics

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What it is

Deliverability is the probability that a sent email reaches the recipient's inbox rather than the spam folder or silent rejection. It is decided per message, per recipient, by the receiving mailbox provider — not by the sender's tooling.

Why it works this way

Mailbox providers optimize for their users, not for senders. Every incoming message is scored on layered signals:

1. Connection and infrastructure — the sending IP's history, reverse DNS, TLS.
2. Authentication — does the message pass SPF, DKIM, and DMARC for the claimed domain?
3. Sender reputation — the provider's running score for the domain and mailbox, built from past behavior. See Sender reputation.
4. Content — links, attachments, HTML weight, spam-pattern phrases.
5. Recipient engagement — do this provider's users open, reply to, and keep messages from this sender, or delete and report them?

The scores combine into a routing decision: accept to inbox, accept to spam, defer, or reject. Providers deliberately keep the exact weights secret and shift them over time.

How it behaves in practice

• Delivery rate ≠ inbox placement. "Delivered" only means the receiving server accepted the message. Placement — inbox vs spam — is invisible in standard delivery metrics and must be measured with seed tests or engagement proxies.
• Reputation is per domain, per mailbox, and per IP — damage to one sending mailbox can spread to its domain.
• Engagement compounds. Replies are the strongest positive signal a cold sender can earn; spam reports are the strongest negative one.
• Volume spikes read as abuse. A mailbox that jumps from 20 to 500 daily sends looks like a compromised account. This is why warm-up exists.
• Thresholds appear at provider policy level: bulk-sender rules (such as Gmail and Yahoo's 2024 requirements) mandate authentication, one-click unsubscribe, and spam-rate ceilings around 0.3%.
• Reputation is not one number. Gmail, Outlook, and every other provider score you from how their users treat your mail, so the same sender can sit in the inbox at one provider and the spam folder at another. Knowing which providers your audience actually uses tells you where to watch.
• Open rates are directional, not exact. Apple Mail Privacy Protection and tighter provider anti-tracking measures mean a reported open is no longer a reliable count of human attention. Open tracking still shows trends — whether mail is landing, being ignored, or drifting toward spam — which makes it most useful during domain warm-up and when diagnosing a sudden drop in performance.
• Compliance is a legal layer, not just a filtering one. CAN-SPAM requires accurate sender information, subject lines that reflect the actual content, clear identification of promotional mail, a visible physical mailing address, and a working opt-out — processed within 10 business days, with the opt-out link kept live for at least 30 days. The sender remains accountable even when a third party sends on their behalf.

Why messages bounce

A bounce is a receiving mail server refusing the message. Hard bounces are permanent rejections; soft bounces are temporary failures. The server states its reason in a Delivery Status Notification (also called a Non-Delivery Report) that lands in the sending mailbox and carries the problem description, the date and time, the rejecting server, an error code, and usually a troubleshooting link. Reply groups the reasons into categories:

The distinction that matters: most categories are about the recipient or the message — an address that no longer exists, an authentication record that does not verify, content a filter dislikes. The two sender's-block categories are about you: the provider decided your sending looks abusive and throttled the account, which is a volume and reputation problem, not a list problem. When Microsoft blocks a sender this way, manual sending from the mailbox usually still works, because the block applies to third-party email automation — so "I can still send from Outlook" is not evidence that the account is healthy.

The thresholds that stop a step, and what a bounce does to the contact record, are defined in bounce handling; how a provider block shows on the sequence toggle and how blocked sends are recovered is in sequence stopping.

What moves open rate

Open rate is a weak measurement of a real effect: whether the message reached a human and looked worth opening. The documented levers, in the order they usually matter:

1. Subject line. Clear and concise — up to 6 words to express the goal of the email. Personalize it with the recipient's name, company, or something relevant to them, spark curiosity or offer value, and A/B test alternatives rather than guessing.
2. Spam triggers. Excessive capital letters and exclamation points ("FREE!!!") and words like "Free" or "Buy now" cost you placement before the subject line ever gets read.
3. List quality. Segment by industry, behavior, and other factors so the message can be customized; remove contacts with no opens in 3–6 months; keep validating addresses to reduce bounces.
4. Send timing. Tuesday and Thursday tend to perform best for B2B audiences, with Wednesday slightly behind; late morning (10–11 a.m.) and early afternoon (1–2 p.m.) are the common windows. Monday mornings, Friday afternoons, and late nights are the documented ones to avoid. For B2C, Saturday mornings and midweek evenings (roughly 7–9 p.m.) can outperform.

None of the timing figures are guarantees — audiences, industries, and geographies behave differently, and A/B testing your own send times is the only way to know. Reply publishes no benchmark open rate to compare against.

Common misconceptions

How Reply implements this

Reply's deliverability capability manages the sender-controlled parts:

• Volume caps. One email account sends at most 400 emails per day on paid plans (200 on trial), inside schedules and the sending-schedule rules.
• Ramp-up by default. Ramp-up mode is active for every newly added email account; the default curve starts at 10 emails per day and adds 3 daily until it reaches 100.
• Human-like pacing. A configurable delay spaces out individual sends; the default account safety setting sends each email with a random delay of 300-450 seconds.
• Warm-up via MailToaster. Peer-network warm-up runs through MailToaster, included with paid subscriptions. See email warm-up.
• Monitoring. A Domain Setup health checker validates the domain's DNS records, and the Spam Monitor tab reads your domain's Gmail spam rate from Google Postmaster Tools — the number to hold below the 0.3% ceiling. Reply's recommended deliverability check via the Mail Tester service is scored from 0 (bad) to 10 (excellent), and Reply recommends custom tracking domains over the default shared ones.
• Automatic stops. Email steps pause for the rest of the day when a sequence's bounce rate crosses 8% (calculated per 24 hours, once at least 200 emails have been sent) — see bounce handling — and replies halt follow-ups per the sequence's reply-handling setting.
• Bounce diagnosis. Bounce reasons surface in three places besides the sending mailbox: hovering the Bounced metric on the Reports page shows a detailed breakdown, the People tab exports the full list of bounce reasons, and a People-tab smart filter narrows contacts to a specific bounce category or provider block.
• Per-provider monitoring. The Contact email provider filter segments prospects by their mailbox provider, so you can review performance against Gmail, Outlook, and the rest separately instead of averaging a number that hides one provider going bad.

Mailbox selection documents how sends are distributed across accounts.

Where this breaks down

No sender-side tool can override a receiving provider's decision. If a domain's reputation is burned, the only fixes are time, reduced volume, and better targeting — or a new domain warmed from zero. Placement testing is also inherently sampled: seed inboxes approximate, but do not equal, your real recipients.

Related

• Sender reputation
• Email warm-up
• Deliverability in Reply
• Why an email was not sent

Build with Reply

• REST API: docs.reply.io — email account settings and limits
• MCP: agents.reply.io/mcp — mailbox health and warm-up operations