All command-line arguments that follow the switches (if any) are taken to be the addresses of recipients. The addresses in the list can be separated by spaces, by commas, or by both:
addr1 addr2 addr3 addr1,addr2,addr3 addr1, addr2, addr3
Certain modes specified by the
switch, such as
-bp (for print the
queue’s contents), cause
sendmail to ignore any list of recipients.
Be sure to escape any characters in addresses that have special
meaning to your shell. For example, because the
character has special meaning to the C shell,
it should be escaped by preceding it with a backslash character:
If sendmail expects a list of recipients and finds none, it prints the following message and exits:
Recipient names must be specified
Note that under some circumstances, sendmail might try to collect the message before issuing this error.
 And its derivatives such as tcsh(1).