A short howto on identifying issues with DKIM not signing 

For example a client had the following error: 

Authentication-Results: REDACTED; dkim=neutral reason="invalid (public key: not available)" header.d=REDACTED

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=REDACTED; 
h=subject:content-type:from:reply-to:to:content-transfer-encoding:cc: content-type:from:subject:to; 
s=REDACTED; bh=REDACTED; b=REDACTED