Plain-text alternative generated for all HTML emails

An HTML-only email displays raw markup in Gmail's push notification preview, in plain-text email clients used in corporate environments, and in assistive technologies that cannot parse HTML. Spam filters from Barracuda, SpamAssassin, and Proofpoint penalize the absence of a multipart/alternative structure because it is a known characteristic of automated spam. RFC-2046 defines the multipart/alternative MIME structure specifically to ensure that all receivers can display a meaningful representation of the message.

Auto-generate the plain-text part from the rendered HTML using html-to-text in the send path:

import { convert } from 'html-to-text'

export function prepareEmailParts(html: string, explicitText?: string) {
  const text = explicitText ?? convert(html, {
    wordwrap: 80,
    selectors: [
      { selector: 'a', format: 'inline', options: { hideLinkHrefIfSameAsText: true } },
      { selector: 'img', format: 'skip' }
    ]
  })
  return { html, text }
}

Pass both html and text to every ESP send call. The text field must not be an empty string — that is treated identically to an omitted field by spam filters.