This is just a quick placeholder so I have a place to point people who claim that Mail.app exports to mbox format via the “Save as…” item in the File menu. It does not. While it’s true that you can select “Raw message source” as an export option, this does not create a valid mbox file. If a message contains a line that starts with the word “from”, the mbox format requires that this line be quoted or otherwise munged (usually by inserting a “>” character at the beginning of the line). Mail.app does not do this, so if you attempt to import the resulting file into another mail program, you will end up with corrupted messages — one that has the proper headers and the first half of the message text, and one with no headers and the second half of the message text. This is a shining example of that pesky data fidelity problem I keep talking about. Planning ahead to reduce the number of migrations is always a better bet than assuming you can migrate your data when the time comes.
However, in the interests of fairness, I will amend my previous statement that Mail.app is a roach motel that auto-upgraded 14 years of my mail into a proprietary, undocumented format with no possibility of exporting it to an open format. This is not true. Mail.app is a roach motel that auto-upgraded 14 years of my mail into a proprietary, undocumented format with a tantalizingly broken export feature. I apologize for the confusion.
(And by the way, I have 66,000 messages in my Mail.app archive, about 1000 of which trigger this bug, so it would be prohibitively tedious to clean up the exported data by hand. Now that I’ve identified the problem, I may have better luck munging the .emlx files in place before running them through Mail.app’s broken export feature. That is, unless there are other lurking surprises that are being hidden by this bug. Stay tuned.)
