One possible form of an address is called "list syntax" and looks like this:

group: list;

Here, group is the name of a mailing list, and list is a list of zero or more addresses to which the message should be delivered. To understand this kind of address, sendmail needs to view the prefix and colon as a comment and the trailing semicolon as a comment. This is similar to treating everything outside an angle-bracketed address as a comment:

group:  list  ;
group: <list> ;

For such addresses to be recognizable, it is necessary to prohibit the use of other addresses that contain colons, unless those colons appear inside a part of the address that is surrounded by angle brackets. That is, to use list syntax, addresses such as the following cannot be allowed:

host:george@wash.dc.gov

To handle this situation, V8.7 sendmail introduced the ColonOkInAddr option. It is used like this:

O ColonOkInAddr=bool               configuration file (V8.7 and later) 
-OColonOkInAddr=bool               command line (V8.7 and later) 
define(`confCOLON_OK_IN_ADDR',bool)    mc configuration (V8.7 and later) 

The argument bool is of type Boolean. If it is absent, this option is true (colons are OK, so list syntax is not recognized). If this option is entirely omitted or if bool is false, colons are not OK, so list syntax is recognized. Note that for version 5 or earlier configuration files (see Section 17.5 for a description of the V configuration command), this option is automatically set to true. Also note that for mc configurations, this option is absent (false) by default.

Note that DECnet-style addresses (Section 19.3.4) legitimately contain double colons (e.g., host::user). DECnet addresses are correctly recognized regardless of how this ColonOkInAddr option is set.

The ColonOkInAddr option is safe. If it is specified from the command line, sendmail will not relinquish its special privileges.