pop - receive mail (POP3 client)

Import with import "pop.j" as pop;. A POP3 receive client (RFC 1939): the line-oriented status dialogue (+OK / -ERR) over the net system library, with plaintext / implicit-TLS / STLS transport and USER / PASS auth. Retrieved messages come back as strings, ready for the mime module to parse. Because it uses net, this module needs the default jennifer binary.

On jennifer-tiny: “needs the default jennifer binary” refers to the stock tiny build, which ships without a network driver - not a TinyGo limitation. A jennifer-tiny rebuilt with a network stack runs this module too; see the note on net and TinyGo.

The module is named pop (not pop3): a Jennifer namespace is letters-only, so a digit in the name can’t be a call prefix. It is POP version 3 - the only one in use - the same choice Ruby’s net/pop makes.

import "pop.j" as pop;
import "mime.j" as mime;

def opts as pop.Options init pop.Options{host: "mail.example.com", port: 995,
    security: "tls", user: "me", pass: "secret"};
for (def raw in pop.fetchAll($opts)) {
    def msg as mime.Part init mime.parse($raw);
    io.printf("subject: %s\n", mime.headerValue($msg, "Subject"));
}

Runnable: examples/modules/pop_demo.j.

Surface

A session is stateful: connect, issue commands, quit. fetchAll wraps the common “get every message” case.

Options.security is "none" (plaintext, port 110), "tls" (implicit TLS on connect, port 995), or "starttls" (STLS upgrade on 110).

Retrieval and dot-stuffing

retrieve and sizes read a multi-line response terminated by a . line, and undo the byte-stuffing POP3 applies (a body line that began with a . was sent doubled, e.g. ..sig on the wire is .sig in the message), so the string you get back is the exact message:

def s as pop.Session init pop.connect($opts);
io.printf("%d messages\n", pop.count($s));
def raw as string init pop.retrieve($s, 1);      # RFC 5322 message text
pop.deleteMessage($s, 1);                          # optional
pop.quit($s);                                      # deletion commits here

A -ERR from the server throws a catchable Error (kind "pop3").

Certificate verification for "tls" / "starttls" is the net default.

Testing

The pure protocol logic - +OK detection, STAT parsing, LIST sizes, and the multi-line dot-terminator / un-stuffing - is unit-tested in the overlay. The networked session is covered end to end by an in-process fake POP3 server in the Go test suite (TestPop3Receive), so it runs in CI without an external server.

Out of scope

• Receive only (retrieve / delete). Sending is smtp.
• USER / PASS, or XOAUTH2 (Options.auth = "xoauth2", via sasl, for Google / Microsoft 365). APOP (MD5 challenge) and the SASL challenge-response mechanisms land with the crypto library.
• No TOP / UIDL. Just STAT / LIST / RETR / DELE.
• An internationalized (IDN) host is IDNA-encoded to its xn-- form automatically (via idna).

Timeouts

Reads carry a 30 s idle timeout (a deadline re-armed before each read), so a hung server fails with a catchable error instead of blocking the caller forever.

See also

• mime.md - parse a retrieved message (mime.parse).
• smtp.md - the send half of the mail suite.
• net.md - the transport pop builds on.
• modules/index.md - the module catalog and import rules.