DKIM canonicalization

What it means

c= names two algorithms, header then body, such as c=relaxed/relaxed. The default is simple/simple, and a single name sets the header algorithm with simple for the body.

Simple header canonicalization changes nothing, so any rewrapping or case change breaks the signature. Relaxed lowercases header names, unfolds lines, collapses runs of spaces and tabs to one space and trims spaces around the colon and at the end. For the body, simple only drops empty lines at the end; relaxed also ignores trailing whitespace and collapses runs of whitespace within lines.

Relaxed survives the reformatting servers routinely apply, so relaxed/relaxed is the safer choice for most senders. Neither survives real content changes such as an added footer or a rewritten link.