Why Your OTP Emails Are Landing in Spam (And How to Fix It)

A one-time password in the spam folder is not just a minor inconvenience, it is a conversion killer. Users who cannot verify their account do not become customers. Users who cannot log in become churned customers. And because the email is time-sensitive, they often give up before checking spam at all. If your OTP emails are landing in spam, the cause is almost always one of four things: missing or broken SPF/DKIM records, a subject line that triggers spam filters, an inconsistent sending pattern, or a shared IP pool with a damaged reputation.

Start with authentication. Every legitimate transactional email sender needs SPF, DKIM, and ideally DMARC configured on their sending domain. SPF tells receiving servers which IPs are allowed to send mail for your domain. DKIM adds a cryptographic signature that proves the message was not tampered with in transit. Without both, Gmail and Outlook will route your mail to spam regardless of how good your content is. PostaSend provides the exact DNS records to add when you verify a domain, it takes about five minutes to configure and propagation completes within an hour.

Subject lines matter more for OTPs than for any other email type because they follow a predictable pattern. Subject lines like Your verification code is 482910 are flagged by some filters as credential-phishing attempts, because that is exactly what a phishing email looks like. Instead, use subject lines that include your product name: Your Example Inc login code or Verify your Example Inc account. Including your brand name also helps users recognize the email as legitimate when they do check spam manually.

Finally, sending consistency matters. If your OTP system sends zero emails for hours and then bursts 500 in two minutes, because a login form was hit by a bot, that burst pattern itself is a spam signal. Implement rate limiting on your OTP endpoint to prevent abuse, and consider adding a CAPTCHA at the login step. A consistent, human-paced sending pattern looks nothing like a spam campaign, and inbox providers notice the difference.