I'm looking for the following features:
- Has a track record of several years
- GUI access to email messages/threads/groups
- GUI access to calender and meetings.
- Stores email messages as separate files in straightforward (Standard?) non-proprietary format
- Stores calender requests as separate files
- Stores email threads/groups/... in directories
- Can search existing email for strings in various fields
- Can merge email files (as above) from another instance.
In particular, I'd like to be able to access the mail messages with arbitrary ad hoc code (e.g, Perl scripts).