There are three main parts of the Unix E-mail system:

Part What It Does Examples
Mail User Agent (MUA) The MUA is the program which the user uses to read and send e-mail. It reads incoming messages that have been delivered to the user's mailbox, and passes outgoing messages to an MTA for sending. elm, pine, mutt
Mail Transfer Agent (MTA) The MTA basicly acts as a "mail router". It accepts a message passed to it by either an MUA or another MTA, decides based upon the message header which delivery method it should use, and then passes the message to the appropriate MDA for that delivery method. sendmail, postfix, qmail
Mail Delivery Agent (MDA) The MDA accepts a piece of mail from an MTA and performs the actual delivery. mail.local, procmail

mail - send email without attachment

mail -s subject < mymailbody.txt

mutt - send email with attachment

mutt -s subject -a /tmp/attachment.txt < mymailbody.txt

Quick - send email test with subject only

echo "subject:hello" | /usr/lib/sendmail -v