My wife forwarded an e-mail as attachment to me, and I could not open it in Claws Mail. The attached e-mail is stored as .eml attachment, and Claws has no way to open it.
Another user had the same problem in 2004, and the answer then was to use the eml2mbox tool. This Ruby-based tool was written in 2004, and it uses a parsedate library function that has been removed in Ruby 1.9. In 2023 we have Ruby 3.1, and eml2mbox does not work anymore.
In the end I used the Python 3 based emlToMbox, which converted the .eml to a .mbox that I could import into Claws.
It worked, but it still sucks that Claws does not open .eml files natively. Forward-as-attachment is a standard procedure.