Why your follow-up emails land in spam (and how to fix it)

1. Authentication: prove you're you

Google's sender guidelines require bulk senders to authenticate outgoing mail with both SPF and DKIM, and to publish a DMARC policy for the sending domain (a policy of p=none satisfies the minimum). Yahoo's sender best practices set out the same expectations: SPF, DKIM, and DMARC with alignment. SPF declares which servers may send for your domain, DKIM cryptographically signs each message, and DMARC tells receivers what to do when the first two disagree.

Google applies these requirements to bulk senders — its documentation uses a threshold of roughly 5,000 messages a day to a Gmail address — but the records cost nothing and help at any volume. Sending from a personal Gmail account while claiming your business domain in the From line is the version of this that fails hardest.

If software sends on your behalf, it should configure these on a dedicated sending domain and show you their live status. Doing it yourself is a single afternoon of DNS edits, once, and your email provider publishes a setup page for each record.

2. Complaints: the one number with a published limit

This is the threshold worth memorizing. Google's sender guidelines instruct bulk senders to keep the spam complaint rate reported in Postmaster Tools below 0.10%, and to avoid ever reaching 0.30% or higher. Yahoo's sender best practices likewise direct bulk senders to stay under 0.30%. Cross those lines and mail starts getting rejected or filtered — and recovery is measured in weeks, not hours.

Two practical consequences. First, install Google Postmaster Tools before your first campaign, not after something breaks; without it you're guessing at the one metric with a published limit. Second, one badly targeted send to a list that doesn't remember you can move that percentage far more than a slow honest campaign ever will.

3. Unsubscribes: one click, and it must actually work

Google requires marketing and subscribed messages to support one-click unsubscribe using the List-Unsubscribe headers described in RFC 8058, plus a clearly visible unsubscribe link in the message body. Yahoo's requirements match, and Yahoo's guidance is that unsubscribe requests should be processed within two days. Transactional mail is outside the one-click requirement.

Separately, and regardless of what any mailbox provider requires, the FTC's CAN-SPAM guidance requires accurate sender identification, a valid physical postal address, and a working opt-out mechanism in commercial email. The mailbox providers can filter you; the FTC can fine you. Satisfy both.

4. Volume: reputation is earned, not assumed

A domain with no sending history that abruptly starts pushing bulk volume looks, to a receiving server, indistinguishable from a spammer warming up — so it gets throttled and filtered. Google's guidance to senders is to increase volume gradually and keep the sending rate reasonably consistent rather than spiking.

In practice that means starting small, watching complaint and bounce signals in Postmaster Tools between increases, and growing as engagement accumulates. If a tool offers to send to your entire list on day one, that offer is telling you something about the tool.

5. List hygiene: dead addresses poison the well

An old CRM export will contain addresses that no longer exist — people change jobs, providers close accounts, and typos were baked in at data entry. Repeated hard bounces damage sender reputation directly. Yahoo's best practices are explicit about monitoring hard and soft bounces and removing invalid recipients promptly.

Before the first send: drop malformed addresses, suppress anything that previously hard-bounced, and honor every prior unsubscribe across every list you hold. Then keep watching your own provider's bounce reporting rather than trusting a single universal threshold — what's tolerable varies by mailbox provider.

6. Content: sound like the person who priced the job

For reactivation, a short message from an identifiable human is usually a better fit than a designed promotional template — it matches what the recipient expects from a contractor they spoke to once. But format alone guarantees nothing. Authentication, complaint rate, consent, relevance, and list quality decide placement; plain text is a reasonable default, not a delivery mechanism.

SignalBack's sending path is designed around these six points — authenticated dedicated domains, ramped volume, automated suppression and hygiene, one-click unsubscribe, and plain sequences. To be clear about status: sending is not built yet, and the free CSV audit is what's live today. When campaigns are enabled, these are intended as defaults rather than toggles.