What Makes an Email Invalid (and Why It Wasn't Sent)

Last updated: May 20, 2026

Every email goes through two automatic checks before it's sent. If an email fails either check, it is flagged as invalid and will not be sent to the recipient.

These checks exist to prevent faulty emails: such as those with broken formatting, unfilled placeholders, or AI-generated artifacts: from reaching prospects. Catching these issues automatically protects sender reputation, keeps outreach professional, and saves you from manually reviewing every message.

This article lists what each check looks for, so you can quickly understand why an email was flagged.

➡ There are two checks, run in order ⬅

Check 1: Pattern Check

Check 2: Content Check

looks for specific text patterns that shouldn't appear in a finished email.

reviews the email content for quality issues.

An email is blocked if it contains:

  • The words null, undefined, or n/a

  • Special characters: [ ] < >

  • A literal newline character (\n) in the text

  • HTML entities such as &amp;, &lt;, &gt;, &quot;, &nbsp;

Forbidden symbols

  • Double dash -- anywhere in the body (including signature separators)

  • Triple asterisks ***

  • Triple backticks ```

  • Triple hash ###

  • Multiple consecutive identical symbols

Empty or broken content

  • The email body is empty

  • Contains the words null, undefined, NaN

Unfilled placeholders

Contains unfilled template tags like <FIRST_NAME>, <COMPANY_NAME>, <ROLE>, or any similar ALL-CAPS placeholder in angle brackets

AI artifacts and meta-commentary

  • Starts with "Unfortunately"

  • Contains phrases admitting lack of context, such as "I do not have enough context" or "I cannot provide a sales message"

  • Contains intro phrases like "Here's an example", "Here is a suggested email", "Following the instructions", or "I can generate a sample email"

  • Contains notes or disclaimers revealing the email is a draft or demo (e.g. "Note: This email is personalized…", "This is a suggested email…")

  • Contains intent-to-write statements (e.g. "I need to create an email to…", "I'll personalize it using their background…")

Missing data admissions

  • Admits lacking case studies, client data, or social proof (e.g. "I don't have public case studies", "no examples were provided")

  • References missing or unavailable input data (e.g. "the inputs didn't provide", "missing context")

Structural issues

  • Duplicate signature or repeated closing phrase at the end of the email

  • Duplicate "Unsubscribe" link or phrase

  • The recipient's name appears more than once in the body

  • The recipient is referred to in third person ("he/she/they") instead of being addressed directly with "you/your"

    E.g. Bad: "Given that he's leading growth at..." / Good: "Given that you're leading growth at..."

  • Mentions an attachment or report but contains no link

  • Contains HTML tags or CSS code

How to Use This

When an email is not delivered, scan the list above to find the matching issue. Pattern-check flags are usually caused by unrendered template variables or stray formatting. Content-check flags usually point to a quality issue worth fixing in the prompt or persona setup.