Mail No attachment, body only -

mailx -m -s "Some Subject" somone@somewhere.com < file_for_email_body

Mail with attachement, no body -
                                                                        
ux2dos /path_to/some_file | uuencode some_file.txt | mailx -m -s "MAIL FROM Someone" someone@somewhere.com

Mail with attachment and body -

ux2dos /path_to/some_file | uuencode some_file.txt | mailx -m -s "MAIL FROM Someone" someone@somewhere.com < file_for_email_body

Mail with carbon and blind carbon -

mailx -m -s "Some Subject" somone@somewhere.com < file_for_email_body

(the file_for_email_body should look like this - )
~c someone_to_cc@somewhere.com
~b someone_to_bc@somewhere.com
The remainder of this file is the text body of the email.  

Mail as another sender

echo "Hello" | mailx -m -s "Greeting" -r Santa.Claus@north_pole.com someone@somewhere.com

Mail with multiple attachments (The sentence "This is all you get." will appear as the body.)

mailx -m -s "Hello" someone@somewhere.com << END                  
`ux2dos /home/some_file | uuencode some_file.txt`    
`ux2dos /home/some_other_file | uuencode some_other_file.txt`                                                                                                                                             
This is all you get.                                               
END