When you send mail to a mailing list that includes your name as a part of that list, V8.10 and later sendmail normally include you in the mailing, whereas V8.9 and earlier normally exclude you. This change in the default behavior was caused by a change in the standards.

The MeToo option overrides the default. The forms of the MeToo option are as follows:

-m                      command-line shorthand 
O MeToo=bool             configuration file (V8.7 and later) 
-OMeToo=bool             command line (V8.7 and later) 
define(`confME_TOO',bool)    mc configuration (V8.7 and later) 
Ombool                   configuration file (deprecated) 
-ombool                  command line (deprecated) 

The optional argument bool, when missing, defaults to true (include the sender). If this option is entirely missing, V8.10 and later default to true (include the sender), but V8.9 and earlier default to false (exclude the sender).

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