In pre-RFC821 days, lists of recipients were commonly space-delimited; that is, the list:

hans christian andersen

was considered a list of three mail recipients, rather than a single, three-part name. Currently, individual recipient names must be delimited with commas, and internal spaces must be quoted. That is:

hans,christian,andersen    three recipients
"hans christian andersen"  a single three-part name
hans christian andersen    illegal

Because some users and some old programs still delimit recipient lists with spaces, the OldStyleHeaders option can be used to tell sendmail to internally convert those spaces to commas.

The forms of the OldStyleHeaders option are as follows:

O OldStyleHeaders=bool              configuration file (V8.7 and later) 
-OOldStyleHeaders=bool              command line (V8.7 and later) 
define(`confOLD_STYLE_HEADERS',bool)    mc configuration (V8.7 and later) 
Oobool                              configuration file (deprecated) 
-oobool                             command line (deprecated) 

The argument bool is of type Boolean. If that argument is missing, the default value is true, and unquoted spaces in an address are converted to commas. The default when configuring with the mc technique is true. If the entire OldStyleHeaders option is missing, it defaults to false, and unquoted spaces are converted to the character defined by the BlankSub option (BlankSub).

The sendmail program is somewhat adaptive about commas. When first examining a list of addresses, it looks to see whether one of the following four characters appears in that list:

, ; < (

If it finds any of these characters in an address list, it turns off the OldStyleHeaders option for the remainder of the list. You always want to enable this option in your configuration file. The only exception might be the unusual situation in which all addresses are normally comma-separated but some legal addresses contain spaces.

Note that comma delimiting allows spaces around recipient names for clarity. That is, both of the following are equivalent:

hans,christian,andersen
hans, christian, andersen

The OldStyleHeaders option is safe. Even if it is specified from the command line, sendmail retains its special privileges.