MIME is documented in RFC2045 through RFC2049.^[60] MIME is a method of incorporating non-ASCII text (such as images and sounds) in mail messages.

^[60] With additional details in RFC2184, RFC2231, RFC2646, and RFC3033.

When sendmail composes an error notification of failed (bounced) mail, this SendMimeErrors option tells sendmail to include MIME-format headers in that error notification. MIME format is required for DSN notification to work (the two go hand in hand). This option affects only returned (bounced) mail.

If the SendMimeErrors option is true and if sendmail is composing a returned mail message, the following two headers are added to the header portion of that message:

MIME-Version: 1.0
Content-Type: multipart/report; report-type=delivery-status;
        boundary=magic

The 1.0 version of the MIME-Version: header (MIME-Version:) is hardcoded into V8 sendmail, so it cannot be changed. The Content-Type: is instead multipart/mixed if sendmail was compiled without DSN support (DSN). The magic of Content-Type: is a string that is used to separate the various parts of the message body. The string is formed from the queue ID, the time, and the hostname. For example:

Content-Type: multipart/report; report-type=delivery-status; 
        boundary="dBPEYdx00413.946132480/your.host.domain"

Then sendmail prefixes the body of the returned message (if there is one), a line of notification, and this boundary:

This is a MIME-encapsulated message

--dBPEYdx00413.946132480/your.host.domain
        message body begins here

Newer MUAs are aware of MIME and can send and receive MIME messages. Such MUAs understand the MIME-Version: header in a mail message. Older (non-MIME aware) MUAs ignore that header.

Unless you bounce mail to a site that cannot handle MIME, you should always set this SendMimeErrors option to true.

The forms of the SendMimeErrors option are as follows:

O SendMimeErrors=bool                  configuration file (V8.7 and later) 
-OSendMimeErrors=bool                  command line (V8.7 and later) 
define(`confMIME_FORMAT_ERRORS',`bool')    mc configuration (V8.7 and later) 
Ojbool                                 configuration file (V8.6 and later) 
-ojbool                                command line (V8.6 and later) 

The optional argument bool is of type Boolean. If bool is missing, this option becomes true (errors are sent in MIME format). If the entire option is missing, it defaults to false (errors are sent just as they were before this option was introduced). The default with the mc configuration technique is to set it to true.

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