Java Multi-Protocol Mail Server

Concurrent Java mail server implementing core SMTP, POP3, and IMAP behavior over TCP sockets, with shared in-memory mailboxes, user authentication, DNS-based inter-domain routing, and a bounded worker pool.

What it demonstrates

• Application-layer protocol implementation from raw socket streams.
• Three different protocol/session models in one server.
• Concurrent client handling with a fixed thread pool.
• Thread-safe shared mailbox/storage access.
• SMTP forwarding based on MX/A DNS resolution.
• Separation between listeners, sessions, protocol handlers, storage, authentication, and routing.

Architecture

                    +------------------+
                    |    MailServer    |
                    +---------+--------+
                              |
             +----------------+----------------+
             |                |                |
        SMTPServer        POP3Server       IMAPServer
             |                |                |
        SMTPSession       POP3Session      IMAPSession
             |                |                |
        SMTPHandler       POP3Handler      IMAPHandler
             |                |                |
             +--------+-------+-------+--------+
                      |               |
                 MailStorage     UserAuthenticator
                      |
                    Mailbox

SMTP forwarding additionally uses DNSResolver.
All listeners submit client work to ServerThreadPool.

Main components

Component	Responsibility
MailServer	Starts and coordinates SMTP, POP3, and IMAP listeners.
SMTPServer / POP3Server / IMAPServer	Accept protocol-specific TCP connections.
*Session classes	Maintain per-client protocol state.
*Handler classes	Parse and execute protocol commands.
MailStorage	Shared mailbox/subscription storage.
Mailbox	Thread-safe collection of email messages.
Email	Message representation with metadata, UID, and flags.
UserAuthenticator	Simple educational authentication model.
DNSResolver	MX/A lookup and routing support.
ServerThreadPool	Bounds concurrent client work.

Build

make

Equivalent command:

javac -d build src/*.java

Run

The original laboratory environment used simulated domains and privileged protocol ports. A typical invocation is:

java -cp build MailServer <domain> <maxThreads>

For local experimentation, you may need to change SMTP/POP3/IMAP ports from 25/110/143 to unprivileged ports and adapt the educational DNS configuration.

The cleaned portfolio copy uses synthetic local accounts alice@<domain> and bob@<domain> with the intentionally simple lab password password.

Concurrency design

• A fixed executor limits the number of concurrent client tasks.
• MailStorage synchronizes shared operations.
• Mailbox synchronizes mutation and retrieval operations.
• Each connection has an isolated session object while sharing the common storage layer.

Important limitations

This is an educational implementation, not a production mail server:

• mailbox contents are memory-only;
• authentication is intentionally simplistic;
• transport is unencrypted;
• IMAP support is partial;
• DNS resolution depends on the system dig command and the original simulated-network addresses;
• no persistent retry queue is implemented for failed forwarding.

Academic context and contribution

Team project with two students. I completed the majority of the implementation and integration work; my teammate also contributed meaningfully across the project. Source-level author attribution is preserved.

Publication status

Prepared for portfolio use. The original report and assignment are not included because they contain academic/submission information. See NOTICE.md.