Mail for RISC OS

A native SMTP, POP3, and IMAP server module for RISC OS, enabling local mail delivery and retrieval.

Overview

Mail is a lightweight mail server implementation designed specifically for RISC OS. It accepts incoming SMTP connections, stores messages locally, and provides POP3/IMAP retrieval with per-user authentication. Recent updates add richer greeting banners with version information, shared authentication helpers, and protocol capability advertising. Perfect for local development, testing, or small-scale deployments on RISC OS systems.

Features

SMTP Server (RFC 5321)

• Listens on port 25 (configurable)
• Domain-based acceptance rules
• Per-user storage organized by domain
• Configurable message size limits
• Custom greeting banner with %HOST% and %VERSION% tokens plus host aliases
• Session timeout management
• Per-mailbox quota enforcement
• EHLO capability advertising (PIPELINING, 8BITMIME, SIZE)

POP3 Server (RFC 1939)

• Listens on port 110 (configurable)
• User authentication from configuration file
• Standard POP3 commands: USER, PASS, STAT, LIST, RETR, DELE, RSET, QUIT
• Extended commands: UIDL, LAST, TOP
• CAPA advertisement including implementation version
• Session timeout management
• Mailbox locking during retrieval

IMAP Server (RFC 3501 subset)

• Listens on port 143 (configurable)
• Shares the POP3 user database for authentication through a common verifier
• Supports core commands: CAPABILITY, LOGIN/LOGOUT, NOOP, LIST/LSUB, SELECT/EXAMINE, SEARCH (ALL/UNSEEN), FETCH/UID FETCH, STORE, EXPUNGE, CLOSE
• Maintains unique identifiers (UIDs) and message flags (\Seen, \Deleted)
• Coordinates mailbox locking with POP3 sessions
• Advertises IMAP4rev1, UIDPLUS, and LITERAL+ capabilities

General

• Daily rotating log files with per-session context
• Real-time session monitoring via *Mail_Status, including listener state and capability summaries
• Hot configuration reload with *Mail_Reload
• Queue status reporting with *Mail_Queue
• Native RISC OS module using SWI calls
• Flexible user aliases with multi-target expansion

Configuration

System Configuration

Edit <Mail$Dir>.Config.System:

Hostname = riscos.local
HostAliases = riscos.local,riscos.test.local
MailboxQuota = 52428800

SmtpEnabled = 1
SmtpListenPort = 25
SmtpSessionTimeout = 300
SmtpBanner = 220 %HOST% Mail %VERSION% ready
MaxMessageSize = 5242880
MaxRecipients = 100

Pop3Enabled = 1
Pop3ListenPort = 110
Pop3SessionTimeout = 300
Pop3Banner = +OK %HOST% POP3 %VERSION% ready

ImapEnabled = 1
ImapListenPort = 143
ImapSessionTimeout = 300
ImapBanner = * OK %HOST% IMAP %VERSION% ready

%HOST% is replaced with the configured hostname, while %VERSION% expands to the module build string logged at start-up. Leave any banner field blank to fall back to the built-in Mail %VERSION% ready text.

Accepted Domains

Edit <Mail$Dir>.Config.Domains (one domain per line):

localhost
riscos.local
test.local

POP3 Users

Edit <Mail$Dir>.Config.Users (format: username@domain password):

# POP3 user accounts
andy@test.local mypassword
user@riscos.local secret123

Note: Passwords are stored in plain text. Protect this file appropriately.

User Aliases

Edit <Mail$Dir>.Config.Aliases to define alternate addresses and forwarding targets:

# alias target1 target2 ...
postmaster@localhost postmaster@riscos.local
andy@riscos.local andy.timmins@riscos.local andrew@riscos.local

Each line lists the alias mailbox followed by one or more delivery targets (local or remote addresses). Aliases are resolved during SMTP delivery; partial matches are case-insensitive. Run *Mail_Reload after editing the file.

Usage

Module Commands

*Mail_Reload    Reload all configuration files
*Mail_Status    Display server status, active sessions, listeners, and capabilities
*Mail_Queue     Show per-mailbox message counts and storage usage

Sending Mail (SMTP)

Connect to port 25 and use standard SMTP commands:

telnet localhost 25
HELO client.local
MAIL FROM:<sender@example.com>
RCPT TO:<user@riscos.local>
DATA
Subject: Test message

This is a test.
.
QUIT

Retrieving Mail (POP3)

Connect to port 110 using any POP3 client or manually:

telnet localhost 110
USER andy@test.local
PASS mypassword
STAT
LIST
RETR 1
DELE 1
QUIT

Checking Mail (IMAP)

Most IMAP clients work out-of-the-box. For manual testing:

telnet localhost 143
A1 CAPABILITY
A2 LOGIN andy@test.local mypassword
A3 SELECT INBOX
A4 FETCH 1 BODY[]
A5 LOGOUT

Directory Structure

<Mail$Dir>/
  ├── Config/
  │   ├── System              Server configuration
  │   ├── Domains             Accepted domain list
   │   ├── Users               POP3 user accounts
   │   └── Aliases             User alias mappings
  ├── !Mail/
  │   ├── !Boot,feb           Module initialization
  │   ├── !Run,feb            Module loader
  │   ├── !Help,fff           User documentation
  │   └── Config/             (Symlinked configs)
  ├── rm/
  │   └── Mail,ffa            Compiled module
  └── c/
      └── mail                Source code

<MailDir$Dir>/
  ├── Storage/
  │   ├── !quota              Quota tracking data
  │   └── domain.user.timestamp    Stored messages
  ├── Logs/
  │   └── YYYYMMDD            Daily log files
  └── Queue/
      └── !manifest           Queue manifest

Building from Source

Requirements

• RISC OS C compiler (DDE)
• CMHG (C Module Header Generator)
• Make utility

Build Steps

1. Set up the build environment:

   amu -f makefile clean
   

2. Compile the module:

   amu -f makefile
   

3. The compiled module will be in rm/Mail,ffa

Build Files

• c/mail - Main source code (SMTP, POP3, IMAP implementation)
• cmhg/modhead - Module header definition
• s/cstart - Module initialization assembly
• makefile - Build configuration
• VersionNum - Version information

Supported Commands

SMTP Commands

• HELO / EHLO - Client greeting (EHLO advertises supported extensions)
• MAIL FROM - Sender specification
• RCPT TO - Recipient specification
• DATA - Message content transfer
• RSET - Clear current transaction
• NOOP - Keep-alive
• QUIT - Close connection
• VRFY / EXPN - RFC compliance (returns "cannot verify")

POP3 Commands

• USER / PASS - Authentication
• STAT - Message count and total size
• LIST [n] - List messages with sizes
• UIDL [n] - Unique-ID listing
• RETR n - Retrieve message
• TOP n lines - Retrieve headers plus specified body lines
• DELE n - Mark message for deletion
• RSET - Clear pending deletions
• LAST - Highest accessed message number
• NOOP - Keep-alive
• QUIT - Close and apply deletions

IMAP Commands (subset)

• CAPABILITY - Advertise IMAP4rev1 UIDPLUS LITERAL+
• LOGIN / LOGOUT - Authenticate and disconnect
• NOOP - Keep-alive
• LIST / LSUB - Enumerate available mailboxes/subscriptions
• SELECT / EXAMINE - Enter mailbox (read-write/read-only)
• SEARCH - Basic search (ALL / UNSEEN)
• FETCH / UID FETCH - Retrieve message data and flags
• STORE - Update message flags (\Seen, \Deleted)
• EXPUNGE / CLOSE - Remove deleted messages

Logging

Logs are written to <MailDir$Dir>.Logs.YYYYMMDD with automatic daily rotation. Each log entry includes:

• Timestamp
• Socket ID (if applicable)
• Event type (SMTP/POP3/IMAP connection, command, error)
• Details (remote address, message content, authentication outcome, etc.)

Security Considerations

⚠️ Important Security Notes:

• No authentication on SMTP (yet) - only deploy on trusted networks
• Plain text passwords in POP3/IMAP - use secure networks or VPN tunnels
• No TLS/STARTTLS support - all traffic is unencrypted
• Domain validation prevents open relay
• Message size and quota limits protect against resource exhaustion
• Session timeouts prevent connection exhaustion

This server is designed for local development and trusted network use only. Do not expose it directly to the Internet.

Troubleshooting

No mail arriving

• Verify the domain is listed in <Mail$Dir>.Config.Domains
• Check port 25 is accessible (*Mail_Status shows listener socket)
• Review logs in <MailDir$Dir>.Logs.YYYYMMDD
• Confirm sender is using correct recipient domain

POP3 or IMAP authentication fails

• Verify user exists in <Mail$Dir>.Config.Users
• Check username includes full email address (user@domain)
• Ensure password matches exactly (case-sensitive)
• Verify domain is also listed in Config.Domains
• Run *Mail_Reload after editing Users file
• Review logs for AUTH entries to confirm the shared helper’s result

Module won't load

• Ensure <Mail$Dir> system variable is set
• Confirm Internet module is loaded (*RMEnsure Internet)
• Check sufficient RMA available (*ROMModules)
• Verify all required directories exist

Configuration changes ignored

• Run *Mail_Reload after editing any config file
• Check file paths are correct (RISC OS uses . as separator)
• Ensure config files are accessible and readable

Out of memory errors

• Reduce MaxMessageSize in System config
• Lower SmtpSessionTimeout and Pop3SessionTimeout
• Check RMA usage with *ROMModules
• Consider reducing MaxRecipients

Technical Details

Message Storage Format

Messages are stored with a Received: header prepended to the original content:

Received: from client.local by riscos.local
	for <user@domain.com>; 20251009145258
Date: Thu, 9 Oct 2025 14:53:35 +0100
From: Sender <sender@example.com>
To: user@domain.com
Subject: Example message

Message body content...

Filenames use the pattern: <MailDir$Dir>.Storage.domain.user.YYYYMMDDHHMMSS

Quota System

• Per-mailbox quotas tracked in <MailDir$Dir>.Storage.!quota
• Quota check performed before accepting messages
• Set MailboxQuota = 0 to disable quota enforcement
• Quotas are per user@domain combination

Event Handling

The module uses RISC OS Internet events:

• Event_Internet (event 19) for socket state changes
• Ticker events for session timeout management
• Callback events for background processing

Contributing

Contributions are welcome! Please ensure:

• Code follows existing style conventions
• RISC OS compatibility is maintained
• Changes are tested on actual RISC OS hardware/emulation
• Documentation is updated accordingly