Found some old mail archives from 1995 to 1999. Several different formats: Unix mailbox, MH folders and RMAIL's BABYL.
Unfortunately, some of the RMAIL files where corruped and some gzipped file were too.
Still, this means I now have ~120 k mail messages in Notmuch. Not bad.
"I appreciate SDF but it's a general-purpose server and the name doesn't make it obvious that it's about art." - Eugen Rochko