Why Marketing Emails Go to Spam and How to Fix Deliverability

The short answer

Mailbox providers decide where your email goes by asking three questions:

1. Is this really you? Checked through SPF, DKIM and DMARC.
2. Can you be trusted? Your domain and IP reputation: complaints, bounces, sending history.
3. Do people want this? Do recipients open, reply or hit "spam"?

Content matters too, but usually less than the first two. If your campaigns suddenly started landing in spam, start with authentication and your list, not with rewriting subject lines.

SPF, DKIM and DMARC in plain words

• SPF lists the servers allowed to send mail on behalf of your domain.
• DKIM is a digital signature: the receiver checks that the message was not altered and was sent by the domain owner.
• DMARC tells receivers what to do with mail that fails the checks and where to send reports. Alignment matters: the domain in the From field must match the domain that passed SPF or DKIM.

All three are DNS TXT records. An example for a domain that sends through an email service provider:

example.com.                 TXT  "v=spf1 include:_spf.esp-example.com ~all"
s1._domainkey.example.com.   TXT  "v=DKIM1; k=rsa; p=MIIBIjANBgkq..."
_dmarc.example.com.          TXT  "v=DMARC1; p=none; rua=mailto:dmarc@example.com"

Your provider gives you the exact SPF and DKIM values. Key details:

• A domain must have only one SPF record; combine several services with multiple include entries.
• SPF has a limit on DNS lookups, so do not add services you no longer use.
• Start DMARC at p=none, read the reports, then tighten to quarantine and reject.

Gmail and Yahoo bulk sender requirements

Since 2024, Gmail and Yahoo enforce mandatory rules for high-volume senders (for Google, roughly 5,000 messages a day to Gmail addresses):

• SPF and DKIM configured and a DMARC record published (at least p=none) with domain alignment;
• one-click unsubscribe via the List-Unsubscribe and List-Unsubscribe-Post headers, with unsubscribe requests honoured promptly;
• a low spam complaint rate: Google sets a threshold of 0.3% and recommends staying below 0.1%;
• valid forward and reverse DNS for sending IPs, and TLS connections.

Even if you send less, meet these rules anyway: they have become the baseline.

Reputation and domain warm-up

Providers do not know a new domain or IP, and a sudden large send looks like spam. Warm-up means increasing volume gradually:

• start with your most engaged subscribers, those who opened and clicked recently;
• raise volume step by step while watching bounces and complaints;
• if complaints rise, hold at the current level.

Using a subdomain for marketing (for example, news.example.com) keeps campaign problems away from your business mail. Google Postmaster Tools helps you monitor reputation with Gmail.

List hygiene

• Remove hard bounces immediately: a non-existent address is a strong negative signal.
• Use double opt-in so you do not collect typos and other people's addresses.
• Send a re-engagement campaign to inactive subscribers and remove those who do not respond.
• Never buy lists: they can contain spam traps, addresses created specifically to catch bad senders.

Content red flags

• Misleading or shouting subject lines: all caps, lots of exclamation marks.
• An email that is a single image with no text.
• Shortened links and links to domains with a poor reputation.
• Heavy attachments instead of links.
• No clear sender, no company postal address, no unsubscribe link.