Outbound Glossary
Plain-text email
A message with no images, tracking or styling. It looks like a person wrote it, and it has the fewest things filters can object to.
Plain text here means a message that carries only words and, at most, one link. No logo, no signature image, no HTML template, no tracking pixel.
Two reasons it performs. Filters score images, link counts and tracking as marketing signals, so removing them removes objections. And a recipient comparing a plain message with a designed one reads the first as correspondence and the second as a campaign. For first touches in outbound this is not a style preference, it is the default.