set sendmail-background

SET SENDMAIL-BACKGROUND YES/no  -  whether to background sendmail

With the usual YES setting, the "sendmail" program runs in the background, 
  and you can proceed to more MM commands.  MM runs sendmail to send the
  message after you give the SEND command at the Send> prompt.

If you change it to NO, you will have to wait for sendmail to finish.
  If you SET SENDMAIL-VERBOSE YES, you may want to wait, so you can see
  messages from sendmail before you continue with MM.

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