Anything that uses standard mbox or maildir formats should be fine. I have mail going back across multiple operating systems and mailer programs since 1995; it

Anything that uses standard mbox or maildir formats should be fine. I have mail going back across multiple operating systems and mailer programs since 1995; it has imported without a problem each time. From 1986-1994 I used a bizarro lash-up to get email through UUCP, which imported the messages into a DOS BBS program to use as a reader. (I said it was bizarro...) The incoming mail was in standard UUCP format, and could have been imported into any common Unix mail program of the day, and thence propagated into the future. Fortunately the BBS mail datasets were plain text, and I can still read them, even if they're not as convenient as a normal mail reader. For that matter, I can still read them running the original 1986 BBS software in DOSEMU.