Apple Mail/DEVONthink almost fail (but is very slow) if there are any messages being imported into DEVONthink with a MIME part of Content-Type: text/enriched
Enriched text was an early markup system before HTML became dominant (as it was envisaged to in the enriched text RFC),.
Although the RFC is dated 1996, I was still receiving messages so encoded in 2018 (i.e. The Late Pre-Covidian Age 
).
See my comments in this thread:
Yet another gotcha in data preservation and portability.
FWIW, I ended up moving those messages out of the mailboxes they were in, editing the source data to exclude the text/enriched MIME part, and then importing then. I didn’t bother converting them to HTML.
If you want to search for such messages, you can export individual Mail mailboxes to .mbox format, then use a text editor to search that file for the Content-Type: text/enriched string.
Feel free to reply or PM if you have any questions, I’m Replying here for posterity 
Sean