It is "Can I successfully reply" to the incomming message. Suppose "bill.clinton@whitehouse.gov" sends you a message.
In response, Spamilter will;

1. Lookup mx hosts for whitehouse.gov
2. Establish a connection to one of the mx hosts listed
3. Simulate a NULL return path message to "bill.clinton@whitehouse.gov" without a body. ie.;

             > 220 wh2.eop.gov -- Server ESMTP (PMDF V5.2-33 #41062)
             < helo my.smtp.host
             > 250 wh2.eop.gov OK, [0.0.0.0].
             < mail from: <>
             > 250 2.1.0 <>... Sender ok
             < rcpt to: <bill.clinton@whitehouse.gov>
             > 250 2.1.5 bill.clinton@whitehouse.gov... Recipient ok
             < quit

4. If a non-"250" result code is returned for the "rcpt to:" statement, then if the result code is a 4XX series code, the the incomming email will be SMFI_TEMPFAIL'd (which returns a 471 error code to the sender), else the incomming email will be SMFI_REJECT'd (which returns a 550 error code to the sender).
5. If a 250 result code is returned, then the email will be proccessed normally by Spamilter/Sendmail.
6. If no connection to a valid mx host is established, then the incomming email will be SMFI_TEMPFAIL'd

From Stéphane Lentz's Milter Introduction try;

• sendmail -d0.13 -bv root | grep MILTER

  for sendmail 8.10.x, 8.11.x, 8.12.x

• sendmail -d0.1 -bv root | grep MILTER

  for sendmail 8.12.x

Lists that automatically Reject or Accept (respectively) an email based on the envelope sender or recipient.

The host name sent during the HELO phase of the MTA negotiation is verified to meet the following criteria;

• hostname is not an ip address
• hostname is not the same as the domain name of the envelope sender address domain
• hostname is resolvable to an ip address