…from the inestimable Dave Edmondson (who is beginning to understand my mindset far too well) I received this suggestion which verges upon practical retrocomputing:

[nail.sourceforge.net]

nail, an enhanced mailx command

Nail is a mail user agent for Unix systems. Highlights are:

* Derived from Berkeley Mail 8.1. An interface like the original Berkeley one is still optionally available.

[…]

* Supports IMAP. In combination with either OpenSSL or Mozilla NSS, IMAPS can also be used. With IMAP, folder hierarchies can be stored and managed on remote servers. This is particularly useful for people who use more than one computer to access their mail. Of course, it is also possible to use IMAP just for fetching mails and storing them locally.

* Supports caching and disconnected operation of IMAP folders. This speeds up operation while in online mode. It also makes it possible to transparently view and edit IMAP mailboxes without an active connection to the server.

…which does sound mighty appealing to someone like me.