Canonicalization

Internet standard. The standardization of email messages formatting that enables consistent hashed values for validating email messages.

To generate the comparison values in metadata like a DKIM-Signature, a sending email server standardizes both headers and body content. This process is called canonicalization. Standards like DKIM follow the Internet Message Format format. Given this standard format, an email message can still undergo some mild modifications in transit. You can choose the level of tolerance you have for these changes as they get authenticated.

If you want to accept common modification, choose relaxed.
If you don't want to tolerate any changes, choose simple.
You can set this value for both the header and body independently, with the header listed first, then a forward slash (/), then the body.
This value defaults to simple/simple.

This results in accepted combinations of simple/simple, simple/relaxed, relaxed/simple, and relaxed/relaxed. If you only provide one value, like relaxed, that value applies to the header. The body gets set to simple and the complete value becomes relaxed/simple.

The service that generates your DKIM signatures sets the method, or algorithm, applied to the header and body content.

On email servers that you manage, you set the canonicalization value in the DKIM configuration file.
Most large scale email servers set their value to relaxed/relaxed.
Twilio SendGrid sets this value to relaxed/simple.

Header canonicalization for DKIM

When writing about canonalization, the following changes get made to email headers:

Algorithm	Changes made
`simple`	None	Keeps headers as byte-for-byte identical to the original.
`relaxed`	Lowercase header names	Converts header names, not values, to lowercase.
	Unfold header lines	Collapses multiline headers into single lines.
	Compress whitespace	Reduces multiple tabs or spaces to single spaces.
	Trim whitespace	Removes spaces at the start and end of lines and after colons.

Body canonicalization for DKIM

When writing about canonalization, the following changes get made to email message bodies:

Algorithm	Changes made	Result
`simple`	Trim blank lines	Ignores blank lines at the end of the message.
`relaxed`	Trim blank lines	Ignores blank lines at the end of the message.
	Compress whitespace	Reduces multiple tabs or spaces within a line to single spaces.
	Trim whitespace	Removes spaces at the end of lines.