What is base64 encoding and when is it bad for deliverability?

Base64 encoding embeds image data directly in the email's HTML rather than referencing external URLs. The image becomes a long string of characters in the source code, eliminating the need for external image hosting.

Deliverability problems: Dramatically increases email size-base64 adds ~33% overhead to file size. Triggers spam filters-many filters flag base64 embedded images as suspicious (it's a technique used to evade URL-based filtering). Blocked by some clients-Gmail strips base64 images entirely. No caching benefits-the full image transfers every time.

Don't use base64 for email images. Host images on a reliable CDN with HTTPS and reference them via URL. This is the standard, expected approach. Base64 might seem clever ("no external dependencies!") but creates more problems than it solves. The deliverability and file size costs aren't worth any perceived benefit.