[MM3-users] What is the recommended format for mailbox interchange as files?

Ralph Seichter wrote:

Having had a shot at writing a robust mbox parser/converter myself, I came across numerous different formats/flavours, and frankly "mbox" is best avoided.

What is the recommended file format for mail archives to share?

I ask because I often see mailman mailing list archives available as some format of mbox file. I like this feature. I want to quickly import a bunch of archived messages into my account so that I can read old threads and get more discussion context anytime I need it.