set sendmail-verbose

SET SENDMAIL-VERBOSE NO/yes  -  whether to see sendmail messages

With the usual NO setting, the "sendmail" program, which MM runs after you
  SEND at the Send> prompt, does not display its diagnostic messages.  The 
  NO setting goes well with SET SENDMAIL-BACKGROUND YES.

If you change it to YES, you probably want to SET SENDMAIL-BACKGROUND NO, 
  so you can track the progress of sendmail.  Normally there is no reason 
  to do so; undeliverable mail will be returned to you in any case.

The variable SEND-VERBOSE is different; it determines whether you see the
  "queued" message for each address, indicating that the mail is being sent.

For more information on sendmail, type "!man sendmail" at any MM prompt, or
  "man sendmail" at the shell prompt.

For more help, type "help" and one of the following:
(at any prompt:)  save-init   set   show   set sendmail-background
                  set send-verbose