Deliverability is mostly authentication and reputation. Both are fixable in an afternoon, and neither is about your subject line.
Spam filters are scoring you, not reading you
Mailbox providers decide where your message lands using a reputation score attached to your sending domain and IP address. Content matters, but it is a smaller input than most people assume, and the word 'free' in a subject line has not been the deciding factor for a very long time.
The inputs that dominate are whether you are cryptographically authenticated, whether recipients engage with your mail, and whether you send to addresses that bounce or file complaints.
Which is good news: all three are things you control.
Authentication: the three records to set today
SPF is a DNS record listing which servers may send mail for your domain. Without it, anything can claim to be you, and receivers treat unauthenticated mail accordingly. Keep it to a single record and watch the ten-lookup limit, which is easy to exceed once you add several tools.
DKIM signs each message with a private key so the receiver can verify it was not altered and did come from you. Your email platform generates the key; you publish the public half in DNS.
DMARC tells receivers what to do when SPF and DKIM fail, and asks them to send you reports. Start at p=none to collect data without affecting delivery, read the reports for a few weeks, then tighten to quarantine and finally reject.
Since 2024 the large mailbox providers have required all three from bulk senders. If any is missing, deliverability problems are expected behaviour rather than bad luck.
List hygiene is the other half
Never buy a list. Purchased lists are full of spam traps, which are addresses maintained specifically to catch senders who did not get permission, and hitting one damages your domain reputation immediately.
Use confirmed opt-in for new subscribers. It costs you some sign-ups and removes the typos, the bots and the people who will report you.
Remove hard bounces at once and suppress them permanently. Continuing to send to invalid addresses is one of the loudest negative signals available.
Sunset the dormant. Someone who has not opened anything in six months is dragging your engagement rate down, and engagement rate is what decides placement for everyone else on your list. Run a re-engagement campaign, then remove the non-responders.
Sending habits that build reputation
Warm up a new domain or IP gradually. Going from zero to fifty thousand sends in a day looks exactly like a compromised account.
Send consistently. A dormant list that suddenly receives a campaign after eight months performs badly, because neither your recipients nor the filters remember you.
Put the unsubscribe link where people can find it, and honour it within two days. Making it hard converts unsubscribes, which are harmless, into spam complaints, which are not.
Use a subdomain for marketing mail, separate from the domain that sends your invoices and password resets. Then a bad campaign cannot take your transactional mail down with it.
Diagnosing an existing problem
Check the DMARC reports first; they tell you whether your mail is authenticating at all. Then check whether the problem is provider-specific, since landing in spam at one provider and the inbox at another points at reputation with that provider rather than a broken record.
Register with the postmaster tools the large providers offer. They expose your complaint rate and reputation directly, which beats guessing.
Look at the last thing that changed. A new sending tool added to SPF, a template change that broke the plain-text version, a list imported from an event: deliverability problems usually have a date.
Our email marketing platform comparison was tested against seeded inboxes at the major providers, and platform choice does move the needle. Mailchimp (9.5/10) and Brevo (9.3/10) both handle authentication setup with the least room for error.
What barely matters
Spam-word checkers. Modern filters are statistical and reputation-led; rewriting 'free' as 'no cost' achieves nothing.
Image-to-text ratios as a hard rule. A sensible balance helps readability, but there is no magic number that flips a filter.
Send-time optimisation, for deliverability purposes. It affects open rates at the margin. It does not affect placement.
Fix authentication and list quality first. If those are wrong, no amount of copy tuning will rescue the campaign.