Delivered is not the same as inbox
An email can leave your platform, get accepted by Gmail's or Yahoo's mail servers, and still never reach the inbox. It can land in Promotions, in spam, or get silently dropped after acceptance. “Delivered” in most platform dashboards means the receiving server accepted the message. It says nothing about where that message ended up. Deliverability is the percentage that actually reaches the inbox, and it is set by authentication, sender reputation and the content of the email itself.
For a peptide brand this is not only a marketing number. If your order confirmation and shipping updates share a domain with your marketing sends, a reputation problem caused by a campaign can take those transactional emails down with it. Keeping a domain and a sending history worth protecting is the point of everything below.
Authentication: SPF, DKIM and DMARC
Three DNS records tell an inbox provider that a message really came from your domain.
- SPF (Sender Policy Framework) lists which mail servers are allowed to send for your domain.
- DKIM (DomainKeys Identified Mail) signs each message with a private key, so the receiving server can verify it was not altered in transit.
- DMARC (Domain-based Message Authentication, Reporting and Conformance) sits on top of both. It publishes a policy telling receivers what to do with mail that fails SPF or DKIM alignment, and it asks receivers to report back on what they saw. As dmarc.org's overview explains, senders otherwise “get very poor feedback on their mail authentication deployments,” and receivers are “wary to reject unauthenticated messages” without that feedback loop. DMARC was built to close that gap: dmarc.org traces the approach back to 2007, when PayPal worked with Yahoo and Gmail on the same sender-receiver collaboration and saw “a significant decrease in suspected fraudulent email purported to be from PayPal.”
None of the three is optional for a bulk sender in 2026. All three are required in some form by both Gmail and Yahoo, covered next.
What Gmail and Yahoo require from bulk senders
Both providers now publish specific thresholds, not general guidance. Quotes below are from each provider's own page, as of 30 September 2026.
Google's bulk sender requirements and Gmail sender guidelines apply once you send close to 5,000 messages or more to personal Gmail accounts within 24 hours, and that status is permanent once you cross it once. Google asks every sender, at any volume, to authenticate with SPF or DKIM, to keep spam rates in Postmaster Tools below 0.30% and to use TLS. Past the bulk line it requires SPF and DKIM on your sending domain, with a DKIM key of 1024 bits or longer for personal Gmail accounts, and a DMARC record at a minimum policy of p=none with the From: domain aligned to SPF or DKIM. Marketing and promotional messages need a one-click unsubscribe: a visible link in the body plus the List-Unsubscribe and List-Unsubscribe-Post headers, which Google points at RFC 2369 and RFC 8058 for. Google's own wording is to “Keep spam rates reported in Postmaster Tools below 0.30%,” and it recommends staying below 0.1%; a sender whose user-reported spam rate is greater than 0.3% is ineligible for mitigation, and becomes eligible again only once the rate stays below 0.3% for seven consecutive days.
Yahoo's sender best practices ask every sender to implement at least SPF or DKIM, and ask bulk senders to implement both, and recommend a DKIM key of at least 1024 bits. Yahoo wants a DMARC policy of at least p=none too, and accepts relaxed alignment. It asks bulk senders for a working List-Unsubscribe header, calls the RFC 8058 one-click method “highly recommended,” and asks senders to honor an unsubscribe within two days. Its own line is to “Keep your spam rate below 0.3%,” measured on mail that reached the inbox.
RFC 8058 adds one more rule on top of both providers' policies: the message carrying the List-Unsubscribe and List-Unsubscribe-Post headers must have a valid DKIM signature that covers both headers, because DKIM is the only sender identity the spec accepts. It also tells senders to put an opaque identifier or another hard-to-forge component in the unsubscribe URI, so nobody else can trigger unsubscribes from your list. That is one more reason DKIM is not optional once you turn on one-click unsubscribe.
Microsoft now draws the same line. Outlook.com's postmaster page says that “Starting May 5th, 2025, Outlook.com is enforcing stricter email authentication standards (SPF, DKIM, DMARC) for domains sending over 5,000 emails per day to enhance security and reduce spam.” That is the same 5,000-a-day threshold Google uses and the same three records, so the DNS work you do for Gmail and Yahoo is the work Outlook.com now checks. Microsoft says mail from a domain that does not meet those standards is routed to the junk folder first, and rejected outright later.
The three policies do not match on every number, so build to the strictest one: SPF, DKIM and DMARC all in place, one-click unsubscribe on every marketing send, and a spam complaint rate kept well under 0.3% rather than close to it.
How to watch your spam rate
Both thresholds above are only useful if you can see the number. Google's sender guidelines FAQ point every sender at Postmaster Tools to check its spam rate, and being above 0.3% even once starts the seven-day clock before mitigation options come back. Yahoo's sender best practices ask bulk senders to enroll in its Complaint Feedback Loop, so complaints on Yahoo mail come back to you directly instead of only showing up as a falling open rate. Check both before you scale a new domain past warm-up, not after a campaign already hurt it.
Your own sending domain vs a shared one
A shared sending domain means every customer on that platform shares the same reputation. One brand with a bad list can drag deliverability down for everyone behind it, and you have no way to see that happening from the outside. Sending from your own domain means your reputation is only ever built or damaged by your own mail. It also means the SPF, DKIM and DMARC records above are records you control, not records that belong to a platform you are borrowing space from. Which platforms will even host a peptide list is a separate question, covered in our policy table for email platforms that allow peptide brands.
Split your mail by type while you are at it. Yahoo on segregating email types tells senders not to send bulk or marketing mail from the same IPs they use for user mail, transactional mail and alerts, and Google on IP addresses and From: lines asks senders who use more than one IP to give each message type its own, with the same From: address across messages of the same category. In practice that means a separate sending subdomain, or a separate IP, for campaigns and for order mail, so a complaint spike on a campaign cannot reach your receipts.
Warm-up: why a new domain or IP starts slow
Inbox providers have no history to judge a brand new sending domain or IP address by, so they start cautious: smaller volume, closer inspection of complaints and bounces, and a slow ramp as the domain proves it sends mail people actually want. Sending your full volume on day one, on a domain with no history, is one of the fastest ways to land in spam before you have sent a single campaign that matters. Neither Gmail's nor Yahoo's policy above carves out a grace period for a new domain: the same 0.3% ceiling applies from message one. Warm-up is how you keep real performance inside that ceiling while the domain is still building the history those providers are watching for. Amino Engine warms up every new sender over 42 days, raising volume gradually as the domain builds a track record with Gmail, Yahoo and every other major provider.
List hygiene: double opt-in, sunsets and bounces
Authentication proves the mail came from you. What you do with your list decides whether providers keep trusting it.
- Double opt-in confirms a subscriber actually wants your mail before you send to them: they confirm through a link in a follow-up email before your regular sends begin, which keeps fake and mistyped addresses off your list from the start.
- Sunset flows stop mailing subscribers who have not opened or clicked in months. Inbox providers read continued sends to unengaged addresses as a signal, not a neutral choice.
- Bounce handlingremoves addresses that hard bounce immediately, rather than retrying them on your next campaign. Yahoo's best practices name this directly: monitor hard and soft bounces, remove invalid recipients promptly, and enroll in a complaint feedback loop where one is offered.
- A visible unsubscribe link in every send. Google asks for a clearly visible unsubscribe link in the message body on top of the one-click headers, and Yahoo tells senders to follow the CAN-SPAM Act wherever in the world they send from, which is where the requirement for a physical postal address in commercial mail comes from.
Amino Engine builds double opt-in into every signup form, popup, flyout, embedded and hosted page alike, so this is the default rather than a setting you have to remember to turn on. The full migration order, export first and switch off last, is in our guide to moving from Klaviyo or Brevo.
The content choices that hurt deliverability
Authentication and list hygiene get you to the inbox door. The content of the email decides whether a recipient reports you once you are there, and a report costs you more reputation than almost anything else on this page. Google's sender guidelines put it plainly: “Over time, user spam reports can lower your domain's reputation.”
- Spammy subject lines: excess punctuation, ALL CAPS, or wording that reads like a filter test.
- Image-only emails with little or no real text, which some filters read as an attempt to hide the actual content.
- Link shorteners, which strip the destination domain that authentication is meant to vouch for.
- Specific claims about a named product, dose or health outcome. Keep copy to neutral placeholders like “Product A, 10mg” rather than naming a peptide next to a benefit. That is also what gets a brand reported by a recipient or flagged by a provider's content filters in the first place.
- Images that say what the text does not: needles, vials against skin, clinical set dressing, before-and-after bodies. Reviewers and image classifiers read the picture as the category.
- Health information stored on a subscriber profile. A survey field, a quiz answer or a segment name built from what somebody told you about their own body is a record that sits in your account whatever your copy says. Keep it off the profile.
Shared vs dedicated sending IP
A dedicated IP puts your reputation on an address only you send from, instead of a pool shared with other senders. That helps once you send enough steady volume to build a reputation on it. Below that volume, a dedicated IP can hurt: an inbox provider sees too little mail to trust, where a shared pool at least has other senders' volume keeping it warm. There is no single volume number where a dedicated IP starts to pay off: it depends on how consistently you send, not just how much. A brand that sends a steady weekly cadence builds reputation on a dedicated IP faster than one that sends in occasional large bursts, even at the same total monthly volume. Google's own bulk sender guidance touches the same tradeoff from the other side: on a shared pool, your reputation sits next to whoever else is sending from the same addresses, for better or worse. Amino Engine's Pro plan ($999/mo, 500,000 emails) includes your own sending IP and a dedicated account manager, for brands that have already reached that volume.
How Amino Engine handles each of these
Here is where each item above lands on Amino Engine. Mail sends from your own domain, on servers and IP addresses we run, and no outside email provider sits above your account. New senders are warmed up over 42 days. Every form type includes double opt-in. Unsubscribes and bounces are tracked and, if you are importing a list from Klaviyo or Brevo, they carry over as unsubscribes and bounces rather than as live contacts. If your volume justifies it, the Pro plan adds a dedicated sending IP and a dedicated account manager.
The full picture of what the platform does is on our email marketing for peptide brands page. Plan details, including email volume by plan: See pricing.
This is general information, not legal advice.