How are HTML and plain text emails structured using MIME?

Most marketing emails include both HTML and plain text versions using MIME's multipart/alternative structure. This ensures readable content regardless of the recipient's email client capabilities.

Structure example:

• Content-Type: multipart/alternative; boundary="----=_boundary"
• ------=_boundary
• Content-Type: text/plain; charset=UTF-8
• [Plain text version]
• ------=_boundary
• Content-Type: text/html; charset=UTF-8
• [HTML version]
• ------=_boundary--

• How it works:
• The boundary string separates each content version
• Parts are listed in order of increasing preference (plain text first, HTML last)
• Email clients choose the "best" version they can display
• Most modern clients show HTML; text-only clients or accessibility tools use plain text
• Why include both:

• Some recipients disable HTML for security
• Screen readers may prefer plain text
• Plain text fallback ensures message is always readable
• Some spam filters check that both versions contain consistent content

ESPs typically auto-generate plain text from HTML, though manual versions often read better. Significant content differences between versions can appear suspicious to filters.