Mail stuck on "Generating content to print"
I see this has been a long-standing problem on earlier versions of Mac OS and Apple Mail. It just happened to me, and I thought I'd share what worked running Mail 11.3 on macOS High Sierra (10.13.4) on a 2017 iMac. I was trying to print some emails from GoDaddy, and kept getting stuck on the "Generating content to print" dialog. This seemed to be specific to mail from GoDaddy; I didn't have any trouble printing messages from other senders.
Following some advice I found in an old post, I went into Mail preferences, and on the Viewing tab, unchecked "Load remote content in messages." After that, printing for messages from this sender Just Worked again. One of the old posts suggested that this was because of hidden pixels embedded in the email, which some senders use and most (in my experience) don't. Seems reasonable, I suppose.