A service business sends a seasonal offer to its whole customer list. Open rates are low, and nobody books. The conclusion is that email does not work. The actual reason is that most of the messages landed in spam, because the list had not been cleaned in three years and the sending domain was never authenticated.

Deliverability is not a mystery. Mailbox providers like Gmail and Yahoo decide where your mail goes based on a small number of signals you control. Here is what they are and what to do about them.

Authenticate the sending domain

Since 2024, Google and Yahoo have published requirements for anyone sending meaningful email volume. The technical core is three DNS records that prove the mail is really from you:

SPF lists which servers are allowed to send mail for your domain.

DKIM signs each message so the receiver can verify it was not altered.

DMARC tells receivers what to do with mail that fails the other two, and sends you reports.

Your email marketing platform and your business email provider each have instructions for setting these up, and whoever manages your domain's DNS can add the records in a few minutes. Without them, your mail is treated as unverified, which in 2026 means it is filtered or rejected. This is the first thing to check if anything is landing in spam.

Separate transactional from marketing sending

Invoices, confirmations, and reminders should not share a sending reputation with promotions. If your marketing list generates complaints, you do not want your appointment reminders following it into the spam folder. Use a separate subdomain or a separate sending service for each stream. The distinction between the two types of message is worth understanding for legal reasons too.

Clean the list

Mailbox providers watch how recipients react. Messages that are ignored, deleted unread, or marked as spam hurt your reputation. Messages that are opened and clicked help. A list full of dead addresses and disengaged contacts drags the whole domain down.

Remove hard bounces immediately. An address that does not exist should never be sent to twice. Any competent platform does this automatically; check that yours does.

Watch the spam complaint rate. Google's requirement is that it stay very low, and they publish the threshold. Every complaint is someone who did not want the message. If the rate rises, stop and find out why before sending again.

Sunset the disengaged. Contacts who have not opened or clicked anything in a long time (a year is a reasonable line for a service business) get one re-engagement message. If they do not respond, stop sending marketing to them. They stay in the CRM with their job history; they just leave the marketing list. Sending to people who never open you is how a domain earns a bad reputation.

Never buy or scrape a list. Purchased addresses have no consent, high complaint rates, and often include spam traps, addresses that exist only to catch senders who did not get permission. One send to a bought list can damage a domain's reputation for months.

Make it easy to leave

A visible, working, one-click unsubscribe is required by the bulk sender rules and is also in your interest. Someone who cannot find the unsubscribe link marks the message as spam instead, which hurts you far more than losing a subscriber. Honor unsubscribes immediately and everywhere.

Send like a business people know

Reputation is built by consistency. A domain that sends nothing for four months and then blasts the whole list looks like a compromised account. A domain that sends a modest volume on a regular cadence looks like a business.

Send from a real name and a real reply-to address that someone reads. "Mike at Smith Plumbing" beats "noreply@." Replies are a strong engagement signal and they are also how customers tell you things.

Write messages people open. The post-job sequence works because each message is useful; a list that only hears from you when you want something disengages, and disengagement is what filters you.

Monitor

Google Postmaster Tools shows how Gmail sees your domain: reputation, spam rate, authentication results. It is free and takes a few minutes to set up. Check it monthly. Your sending platform's own deliverability reports (bounces, complaints, opens by mailbox provider) fill in the rest.

If Gmail opens drop sharply while others hold, you have a Gmail reputation problem. If everything drops at once, check authentication first.

When it is already bad

If your domain has been filtered, the recovery is slow and boring: fix authentication, cut the list to the engaged contacts only, send small volumes of genuinely useful messages for several weeks, and let the reputation rebuild. Switching to a brand-new domain to escape the problem is a known spammer pattern and is treated as one.

Authentication and stream separation are set up as part of the email marketing work we run, and the list lives in the CRM where engagement can be tracked.

Frequently asked questions

How big a list counts as bulk sending?

Google's published threshold is in the thousands of messages per day to Gmail addresses. Most service businesses are below it, but the same practices apply at any volume; the rules simply become mandatory rather than advisable.

Should I clean the list before a big send or after?

Before. A big send to a dirty list is what triggers the reputation hit.

Is it worth using a verification service on old addresses?

For a list that has not been sent to in a year or more, yes. It removes the dead addresses without risking a bounce spike.

What about text messages? Same problem?

Different mechanism, similar principle. Carriers filter SMS based on registration and complaint patterns, and what you can send is governed by consent rather than engagement.