Mailutils debugging output is controlled by a set of levels, each of which can be set independently of others. Each debug level consists of a category name, which identifies the part of Mailutils for which additional debugging is desired, and a level number, which tells Mailutils how verbose should its output be.
Valid debug levels are:
Implementation and applicability of each level differs between various categories. The full list of categories is available in file libmailutils/diag/debcat in the Mailutils source tree. Most useful categories and levels implemented for them are discussed later in this article.