POP3 client may not detect server's APOP capability

[jpmerkel]

This is a minor issue, considering APOP is so old and outdated, but I noticed APOP did not work with my mail server. The code in Pop3Engine.cs expects the <timestamp> to appear right after the +OK in the server's greeting, but that is not required per RFC 1939. The example on page 16 of the RFC shows it at the end of the line:

S: +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us>
C: APOP mrose c4c9334bac560ecc979e58001b3e22fb
S: +OK maildrop has 1 message (369 octets)

Here is a snippet from the MailKit protocol log from my test:

Connected to pop://10.20.42.23:110/?starttls=when-available
S: +OK mail.test POP3 ready <F201706231210.AA102635MD1932@mail.test>
C: CAPA
S: +OK Capability list follows
S: TOP
S: USER
S: UIDL
S: .
C: USER frank@mail.test
S: +OK frank@mail.test... User ok
C: PASS *********
S: +OK frank@mail.test's mailbox has 1 total messages (4826 octets)

It went straight to USER without attempting APOP.

Changing the code in Pop3Engine.cs from this:

            index = text.IndexOf('>');
            if (text.Length > 0 && text[0] == '<' && index != -1) {
                ApopToken = text.Substring(0, index + 1);
                Capabilities |= Pop3Capabilities.Apop;
            }

to this:

            index = text.IndexOf('<');
            if (index != -1) {
                int index2 = text.IndexOf('>', index + 1);
                if (index2 != -1) {
                    ApopToken = text.Substring(index, index2 - index + 1);
                    Capabilities |= Pop3Capabilities.Apop;
                }
            }

fixes it for me.

Here's a protocol log snippet afterwards:

Connected to pop://10.20.42.23:110/?starttls=when-available
S: +OK mail.test POP3 ready <F201706231403.AA0326161MD4798@mail.test>
C: CAPA
S: +OK Capability list follows
S: TOP
S: USER
S: UIDL
S: .
C: APOP frank@mail.test 22f8e98cabe32c27fa273f049ae13a61
S: +OK frank@mail.test's mailbox has 1 total messages (4826 octets)

[jstedfast]

Thanks.