mail.tcl

#!/usr/bin/expect
set timeout 10
spawn telnet mail.example.com 25
expect {
    default {exit 2}
    "ESMTP Postfix"
}

# helo
send "HELO sender.example.com\n"
expect {
    default {exit 2}
    "postfix.example.com"
}

# envelope-from
send "MAIL FROM: <my@sender.example.com>\n"
expect {
    default {exit 2}
    "250"
}

# not exists
send "RCPT TO: <not_found@example.com>\n"
expect {
    default {exit 2}
    "rejected"
}

# Third-Party Mail Relay
send "RCPT TO: <other_domain@other.example.com>\n"
expect {
    default {exit 2}
    "denied"
}

# ok
send "RCPT TO: <yamasita@example.com>\n"
expect {
    default {exit 2}
    "Ok"
}

send "QUIT\n"
interact
puts [exec echo Finished]