First each of two mailing lists are expanded in separate sessions with different hosts. Then the message is sent to everyone that appeared on either list (but no duplicates) via a relay host.
Step 1 -- Expanding the First List R: 220 MIT-AI.ARPA Simple Mail Transfer Service Ready S: HELO SU-SCORE.ARPA R: 250 MIT-AI.ARPA S: EXPN Example-People R: 250-<ABC@MIT-MC.ARPA> R: 250-Fred Fonebone <Fonebone@USC-ISIQ.ARPA> R: 250-Xenon Y. Zither <XYZ@MIT-AI.ARPA> R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA> R: 250-<joe@foo-unix.ARPA> R: 250 <xyz@bar-unix.ARPA> S: QUIT R: 221 MIT-AI.ARPA Service closing transmission channel Step 2 -- Expanding the Second List R: 220 MIT-MC.ARPA Simple Mail Transfer Service Ready S: HELO SU-SCORE.ARPA R: 250 MIT-MC.ARPA S: EXPN Interested-Parties R: 250-Al Calico <ABC@MIT-MC.ARPA> R: 250-<XYZ@MIT-AI.ARPA> R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA> R: 250-<fred@BBN-UNIX.ARPA> R: 250 <xyz@bar-unix.ARPA> S: QUIT R: 221 MIT-MC.ARPA Service closing transmission channel Step 3 -- Mailing to All via a Relay Host R: 220 USC-ISIE.ARPA Simple Mail Transfer Service Ready S: HELO SU-SCORE.ARPA R: 250 USC-ISIE.ARPA S: MAIL FROM:<Account.Person@SU-SCORE.ARPA> R: 250 OK S: RCPT TO:<@USC-ISIE.ARPA:ABC@MIT-MC.ARPA> R: 250 OK S: RCPT TO:<@USC-ISIE.ARPA:Fonebone@USC-ISIQA.ARPA> R: 250 OK S: RCPT TO:<@USC-ISIE.ARPA:XYZ@MIT-AI.ARPA> R: 250 OK S: RCPT TO:<@USC-ISIE.ARPA,@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA> R: 250 OK S: RCPT TO:<@USC-ISIE.ARPA:joe@FOO-UNIX.ARPA> R: 250 OK S: RCPT TO:<@USC-ISIE.ARPA:xyz@BAR-UNIX.ARPA> R: 250 OK S: RCPT TO:<@USC-ISIE.ARPA:fred@BBN-UNIX.ARPA> R: 250 OK S: DATA R: 354 Start mail input; end with <CRLF>.<CRLF> S: Blah blah blah... S: ...etc. etc. etc. S: . R: 250 OK S: QUIT R: 221 USC-ISIE.ARPA Service closing transmission channel Scenario 7