Guide
9 min read
Deliverability is the foundational problem of cold email. The best copy in the world does nothing from the spam folder. Understanding what drives inbox placement — and what undermines it — is the difference between an outbound operation that performs and one that looks like it is performing while most of its emails are silently discarded.
Deliverability is not a single setting you enable. It is the cumulative result of your domain reputation, your sending behaviour, your list quality, and your content. Improving it requires attention to all four.
Every domain and IP address that sends email has a reputation score maintained by major email providers — primarily Google and Microsoft, who together handle the majority of business email. This score reflects how recipients interact with emails from your domain: do they open them, do they reply, do they mark them as spam, do they ignore them?
A high reputation score means emails from your domain are trusted and delivered to the inbox. A low score means emails are filtered to spam or blocked entirely. The score changes based on sending behaviour — it can improve over months of responsible sending or degrade rapidly after a bad campaign.
Authentication records are the technical foundation of deliverability. SPF tells receiving servers which mail servers are authorised to send email from your domain. DKIM adds a cryptographic signature to every email, proving it was sent by an authorised source and has not been altered. DMARC tells receiving servers what to do when an email fails these checks and enables you to receive reports on your domain's sending activity.
All three must be configured correctly for every sending domain. Missing or misconfigured authentication is the single most common cause of spam placement for outbound teams that otherwise have good sending practices. Check your configuration using MXToolbox or Google's free postmaster tools.
Sending to a dirty list — one with a high proportion of invalid, inactive, or role-based email addresses — damages reputation faster than almost any other factor. Every hard bounce (an email rejected because the address does not exist) is a signal to receiving servers that you are not maintaining your lists responsibly. Accumulate enough bounces and your domain's reputation drops.
Validate every email address before it enters a sending sequence. Tools like NeverBounce, ZeroBounce, or Bouncer check whether an email address is likely to be valid before you send to it. A validation step adds cost and time but saves significantly more by protecting domain reputation.
Spam filters analyse the content of emails alongside sender reputation. Content patterns that trigger spam filters include: excessive links (more than one or two in a cold email), HTML formatting that resembles a marketing template, trigger words associated with spam ("free," "guaranteed," "no obligation," "act now"), and mismatches between the displayed link text and the actual URL.
Cold emails should be plain text. No images, no HTML templates, no formatted headers. The email should look like a message written by a person in an email client, because that is exactly what spam filters are looking for. A plain text email with specific, relevant content from a trusted domain is the combination most likely to reach the inbox.
Sudden changes in sending volume are a red flag for spam filters. A domain that sends ten emails on Monday and 500 on Tuesday looks like a compromised account. Volume should increase gradually — a ramp that mirrors what a person ramping up their outreach would look like, not a switch that flips from zero to scale overnight.
Sending at consistent times, within normal working hours, with natural variation in the timing between sends, produces a behavioural profile that looks human. Automation tools that send at regular intervals — exactly 30 seconds between each email, for instance — look automated because they are. Use tools that introduce randomised delays.
Monitor deliverability continuously, not just when you notice a problem. Key metrics: open rate (declining open rates often precede spam placement), bounce rate (should stay below 3%), spam complaint rate (should stay below 0.1%), and placement test results from a tool like GlockApps.
If you detect a deliverability problem — spam placement on a significant portion of sends — stop campaign sending from the affected domains immediately. Investigate the cause before resuming. Continuing to send while investigating compounds the damage. A domain that is partially in spam will degrade further if you keep sending at the same volume.