MH is a command-line mail utility for the Unix environment.
With this setup, mh fits right into the normal Unix environment of software tools. You can use the normal unix tools for message processing and searching. The unix shell is your mail handling facility, with all of its scripting, variables, aliases, and other tools available to provide an uber-rich mail environment. Another advantage is that emails are immediately accessible without any overhead of starting a mail program -- even if your mail has gigabytes of data!
mh was maintained for years, and then a new effort nmh took over the underlying tool set.
GUI tools are also available for the X11 enviroment; xmh is a basic X11 interface to the MH environment. An newer alternative is the exmh, a extensible X11 MH application, which is written in TCL/Tk.
emacs has a MH processing mode, mh-e which is another way to access mh functionality in a GUI environment. The displays can be emacs buffers, or executables that are run by emacs to display content.
Although console based, MH has facilities for handling of non-text emails -- from the text level. The mhn facility of MH allows creation and access of MIME messages from the command line. Non-text content can be viewed by an apprporiate per-domain viewer; for example running xtern on X11 you can read text emails as normal ... and MH can also start a image or document processor to display graphical content.
I use pop3 or imap email access on a few clients, such as smart phones and other devices. The problem with reading email on these platforms is that ... the status of messages in your actual inbox is marked as unread. Also, replies and other actions are not recorded in the normal MH status and database.
This has led to a duplication of email handling, which ... takes more time than it used to do. I would like to integrate pop and imap message handling into MH, so that I can access my normal mail archives from my portable devices. That is a bit of a problem: the many remote mailbox access protocol -- and the clients which use them -- don't let you perform rich actions on your own emails. They also have incompatible formats of message processing. That's my dream for unified mail handling; I hope I have time to do something about it one of these days.