As with paper mail, you need to know the address. To send mail to someone on the same host as you, you can just type the userid, as shown in the example here. To send to someone on another host, you need to type the address in the form userid@host. You can see an address in that form in the From field of a message you've read.
Start by typing send or just s at the MM> prompt:
MM>send To:
Four prompts will appear, one at a time:
MM>send To: jb51 cc: [return] Subject: Meeting Sept 21 Message (End with CTRL/D or ESC Use CTRL/B to insert a file, CTRL/E to enter editor, CTRL/F to run text through a filter, CTRL/K to redisplay message, CTRL/L to clear screen and redisplay, CTRL/N to abort, CTRL/P to run a program and insert output.):
After the long Message... prompt, start typing your message. When you finish the message, signal MM by pressing either escape or control-d.
While you are typing the message, if you make an error, you can backspace to it (even to a previous line), and then retype forward.
MM>send To: jb51 cc: [return] Subject: Meeting Sept 21 Message (End with CTRL/D or ESC Use CTRL/B to insert a file, CTRL/E to enter editor, CTRL/F to run text through a filter, CTRL/K to redisplay message, CTRL/L to clear screen and redisplay, CTRL/N to abort, CTRL/P to run a program and insert output.): OK, I can meet then. --Fuat [escape] [ D=display S=send TE=text ED=edit TY=type Q=quit ?=Hints HELP ] S>
At the S> prompt, you can decide whether to send this message. To send it, type send or s:
S>send jb51... Queued MM>
You might decide not to send the message after all. Type quit or just q to discard the message.
S>quit MM>