One difference between X.400 and Internet Mail is the way they are packaged: in X.400, it is common to buy an MTA and its associated UAs as one package, although the market for separate UAs using the P7 interface is growing; several companies now sell UAs that will work with any MTA supporting P7 (with a reasonable profile).
In the Internet mail marketplace, it is quite common to sell
only UA products; the MTA marketplace is dominated by the
"freeware" Sendmail, which is a quite capable mail handler, and some
large niches carved out by speciality MTAs; most of them involve some
kind of gatewaying, or run on non-UNIX operating systems.
If the MTAs run on Unix at all, they will provide a Sendmail-like
programmatic interface (/usr/spool/mail and the sendmail command), and
most probably a POP or IMAP server for remote UA access. This makes
changing UAs easy.
The freeware market for UAs is also at the forefront of technology, seeing such products as EXMH and Pine, which seem to be surprisingly well backed by their respective developers. What they are not is well tested and stable; the expression "bleeding edge technology" seems particularly apt for these producgts.
There is no "freeware" market in X.400 products. This may be because writing an X.400 product is an inherently expensive project, or it may be because at the time "everyone" thought X.400 was going to be a success, "everyone" thought that they could earn money from implementing it, so they never released anything as freeware.
When it gets down to numbers, the X.400
product list currently lists approximately 49 products, most of
which are MTA or gateway offerings, some of which have UA components.
There are surely more X.400 products out there.
For Internet mail, I don't have a number to point at. Any list would be off the top of my head. (OK, the top of my head contained 19 entries....the FAQ of the comp.mail.misc newsgroup probably holds as many more)