1. An implementation of MIME. Metamail is an implementation of MIME (Multi-purpose Internet Mail Extensions), a proposed standard for multimedia electronic mail on the Internet. Metamail is configurable and extensible via the "mailcap" mechanism described in an informational RFC that is a companion to the MIME document. Metamail can be used to turn virtually any mail reader program into a multimedia mail reader. For information about how to change mail readers so that they can use Metamail, please read the file `/usr/share/doc/metamail/mailers.txt.gz'. From Debian 3.0r0 APT

2. Metamail is a system for handling multimedia mail, using the mailcapfile. Metamail reads the mailcap file, which tells Metamail what helper program to call in order to handle a particular type of non-text mail. Note that metamail can also add multimedia support to certain non-mail programs. Metamail should be installed if you need to add multimedia support to mail programs and some other programs, using the mailcap file. From Mandrake 9.0 RPM