[SwellJoe]

mbox and Maildir have both been around for years, and will be around for years to come. Both are easily converted into each other, and there are many common tools for reading and writing them. Most major languages have APIs for working with either spool type.

mbox has been around for at least 20 years. I don't know if it will be around for 30 years, but it's such a simple format, that I'm sure you could deal with it in 30 years even if no tools still commonly use it--you can open it in a text editor and make sense out of it without any parsing, at all, for example. (MIME encoded messages are much harder to parse, and can contain infinite other formats, but the actual email is always gonna be easy to get at.)