When a mail message is converted from 8 to 7 bits (see the EightBitMode option in EightBitMode), it is important that the result look like a MIME message. V8.7 and later sendmail first outputs the following header if one is not already present:

MIME-Version: 1.0

After that, sendmail looks for a Content-Type: header (Content-Type:). If none is found, the following is inserted, where dfltchset is the value declared for this option:

Content-Type: text/plain; charset=dfltchset

The forms of the DefaultCharSet option are as follows:

O DefaultCharSet=dfltchset               configuration file (V8.7 and later) 
-ODefaultCharSet=dfltchset               command line (V8.7 and later) 
define(`confDEF_CHAR_SET',dfltchset)     mc configuration (V8.7 and later) 

If the DefaultCharSet option is undefined, dfltchset defaults to the string unknown-8bit. The default for the mc technique is to omit this option.

Note that if the C= equate (C=) is present for the sender's delivery agent, that character set supersedes this DefaultCharSet.

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