Email accessibility applies familiar web principles, meaningful heading structure, descriptive alt text, adequate color contrast, logical reading order, to a medium with its own unique constraints: email clients render HTML and CSS inconsistently, and many still rely on table-based layouts that need careful markup to remain readable to a screen reader.
Common failures include image-only promotional emails with no real text at all, low-contrast branded color schemes carried over from a print style guide, and links that all say Click Here with no distinguishing context for someone navigating by link list.