I think that end-users who are not very brave hackers would appreciate if
they could simply select "SpamAssasin" from a list of "Mail filterng
software installed on your computer" instead of typing its command line
(error-prone and not straightforward).

- - -

For that, we could create a standard directory (like
/usr/share/evolution/external-filters/), where every filtering program
would place its description file upon installation.

The description file would be XML-formatted and contain the command to run
(e.g. "spamc"), a "friendly name" ("SpamAssasin") and a description ("This
filter recognizes unsolicited mail..."). The last two items should be
i18n-able.

Every filtering application would have a separate file, which'll be shipped
with its package (without depependency on "evolution"), so that simple
package management could install and uinstall the Evolution integration.

- - -

At some later point, this directory could be standardized accross mailers,
but I don't see it happening now.