POP3server

An educational POP3 (RFC 1939) server and console client written in C++ with WinSock for Windows. The server keeps a few in-memory test accounts with demo mail and walks each client through the three protocol states:

AUTHORIZATION --USER/PASS--> TRANSACTION --QUIT--> UPDATE

Warning — for learning only. Authentication is plaintext USER/PASS over an unencrypted TCP connection, accounts are hardcoded and mail lives in process memory. Never expose this server to a real network or reuse real passwords with it.

Projects in the solution

Project	Binary	Purpose
Project21	Project21.exe	Multi-threaded POP3 server
POP3client	POP3client.exe	Interactive console POP3 client

Building

Requirements: Visual Studio 2022 (v143 toolset) with the "Desktop development with C++" workload and a Windows 10/11 SDK.

• IDE: open Project21.sln and build the Release|x64 configuration (any of the four configurations works).
• Command line:

"C:\Program Files\Microsoft Visual Studio\2022\Community\MSBuild\Current\Bin\MSBuild.exe" Project21.sln /p:Configuration=Release /p:Platform=x64

Binaries land in x64\Release\ (or Debug, Win32 variants).

Running the server

x64\Release\Project21.exe 8110

The single optional argument is the listening port (1–65535, default 8110). Each client connection is served on its own thread.

Running the client

x64\Release\POP3client.exe 127.0.0.1 8110

Both arguments are optional (host defaults to 127.0.0.1, port to 8110). Type commands at the pop3> prompt:

+OK POP3 server ready
pop3> USER wladez
+OK wladez is a valid mailbox
pop3> PASS password
+OK wladez's maildrop has 2 messages (143 octets)
pop3> LIST
+OK 2 messages (143 octets)
1 67
2 76
pop3> RETR 1
+OK 67 octets
From: lera
To: wladez
Subject: Just for fun

Hi! How are you?
pop3> QUIT
+OK POP3 server signing off (2 messages left)

Any telnet-style client works as well: telnet 127.0.0.1 8110.

Test users

Login	Password	Mailbox
wladez	password	2 demo messages
azat	12345	empty
lera	1q2w3e	empty

Supported commands

Command	State	Description
USER x	authorization	Select a mailbox name
PASS x	authorization	Authenticate; locks the maildrop on success
STAT	transaction	Message count and total size in octets
LIST [n]	transaction	Scan listing for all messages or message n
RETR n	transaction	Retrieve message n
DELE n	transaction	Mark message n as deleted
RSET	transaction	Clear all deletion marks
NOOP	transaction	Do nothing, reply +OK
CAPA	any	Capability list (RFC 2449)
QUIT	any	From transaction: enter UPDATE — purge marked mail

Deletions are only applied when the session ends with QUIT (the UPDATE state). A dropped connection rolls the marks back, as the RFC requires. Only one session may hold a maildrop at a time; a second login gets -ERR unable to lock maildrop.

Tests

With the solution built (Release|x64):

powershell -ExecutionPolicy Bypass -File tests\smoke-test.ps1

The smoke test starts the server on a spare port, drives the full protocol scenario over raw TCP (including octet counts, error paths, deletion persistence across sessions and the maildrop lock) and runs the console client with scripted input. It exits non-zero if any check fails.