It seems to be a hit or miss. A few days ago I dragged and dropped from Apple Mail into a Group in DTP, it worked with no issue.
Today I tried copying a short thread (also tried individually) and received error messages. “Import of 2 messages failed”, then “Not Connected to Internet. CK ErrorDomain 3” (yet I am connected, as I type this and read articles on the web). They might be unrelated, as the latter message keeps repeating.
I don’t want to do any bulk email processing as I’ve seen some like to do, just import a few specific emails to DTP.
I tried quitting and restarting both apps, no change.
Sonoma 14.7.1 on a M3 MacBook Pro.