Comments provide you with the documentation necessary to maintain the configuration file. Because comments slow down sendmail by only a negligible amount, and only at startup, it is better to overcomment than to undercomment.

Blank lines and lines that begin with a # character are considered comments and are ignored. A blank line is one that contains no characters at all (except for its terminating newline). Indentation characters (spaces and tabs) are invisible and can turn an apparently blank line into an empty-looking line, which is not ignored:

# text			 a comment
tabtext a continuation line 
			 a blank line
tab an "empty-looking line" 

Except for two special cases, pre-V8 comments occupy the entire line. The two special cases are the R and S configuration commands. The R command is composed of three tab-separated fields, the third field being a comment that does not require a leading # character:

Rlhs      rhs       comment

The pre-V8.7 S command looks only for a number following it and ignores everything else, so it can also be followed by a comment:

S3 this is a comment

Prior to V8, no other commands allow comments to follow on the same line:

CWlocalhost mailhost  # This won't work prior to V8

    Part I: Build and Install
    Part II: Administration
    Part III: The Configuration File
    Chapter 21. The D (Define a Macro) Configuration Command
    Chapter 24. The O (Options) Configuration Command