String welcomeMessage = client.getReplyString();
// check for valid syntax that include all info needed for APOP
assertTrue(welcomeMessage.trim().matches("\\+OK \\<\\d+\\.\\d+@.+\\> .+"));
int reply = client.sendCommand("APOP invalid invalid");
assertEquals(POP3Reply.ERROR, reply);
handler.add("valid", new MockMailbox("id"));
reply = client.sendCommand("APOP valid valid");
assertEquals(POP3Reply.OK, reply);