Initial commit: IMAP email downloader

Single-file Python script to download emails from IMAP servers:
- Downloads emails as .eml files preserving folder structure
- Extracts attachments to zip files
- Supports SSL and STARTTLS connections
- Incremental updates using UID tracking (default behavior)
- Multi-account support with separate folders per email
- Safety checks to prevent duplicate downloads

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plan.md

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+# Implementation Plan: IMAP Downloader
+
+## Overview
+
+Create a single-file Python script (`imapdown.py`) that downloads all emails from an IMAP server and saves them as individual EML files in a local folder structure mirroring the IMAP mailbox hierarchy.
+
+## Implementation Steps
+
+### 1. Argument Parsing
+
+Use `argparse` to handle command line arguments:
+
+**Mandatory arguments:**
+- `--server` - IMAP server hostname
+- `--email` - Email address
+- `--user` - Username for authentication
+- `--password` - Password for authentication
+
+**Optional arguments:**
+- `--ssl` - Use implicit SSL/TLS (typically port 993)
+- `--starttls` - Use STARTTLS upgrade (typically port 143)
+- `--port` - Custom port (defaults: 993 for SSL, 143 for STARTTLS/plain)
+
+Add mutual exclusion for `--ssl` and `--starttls`.
+
+### 2. IMAP Connection
+
+- Use Python's built-in `imaplib` module
+- Connection logic:
+  - If `--ssl`: Use `IMAP4_SSL` (default port 993)
+  - If `--starttls`: Use `IMAP4`, then call `starttls()` (default port 143)
+  - If neither: Use plain `IMAP4` (default port 143)
+- Authenticate with provided credentials
+
+### 3. Folder Discovery
+
+- Use `list()` method to get all mailbox folders
+- Parse folder names and hierarchy delimiter
+- Handle folder name encoding (IMAP uses modified UTF-7)
+
+### 4. Email Download
+
+For each folder:
+1. Create corresponding local directory structure
+2. Select the folder with `select()`
+3. Search for all messages with `search(None, 'ALL')`
+4. For each message:
+   - Fetch the complete RFC822 message
+   - Generate a unique filename (using UID or message ID + date)
+   - Save as `.eml` file
+
+### 5. File Naming Strategy
+
+Use a naming scheme that ensures uniqueness and provides useful info:
+- Format: `{UID}_{date}_{subject_snippet}.eml`
+- Sanitize subject for filesystem safety
+- Handle duplicates by appending counter if needed
+
+### 6. Error Handling
+
+- Connection failures
+- Authentication errors
+- Folder access issues
+- Invalid/corrupt messages
+- Filesystem errors (permissions, disk space)
+
+## Dependencies
+
+Only Python standard library:
+- `imaplib` - IMAP protocol
+- `argparse` - Command line parsing
+- `email` - Email message parsing
+- `os` / `pathlib` - Filesystem operations
+- `re` - Regex for sanitization
+- `datetime` - Date handling
+
+## Output Structure
+
+```
+./download/
+├── INBOX/
+│   ├── 1_20240115_Meeting_notes.eml
+│   └── 2_20240116_Project_update.eml
+├── Sent/
+│   └── 1_20240114_RE_Question.eml
+└── Archive/
+    └── 2023/
+        └── 1_20230501_Old_email.eml
+```
+
+## Testing Approach
+
+1. Test argument parsing with various combinations
+2. Test connection with SSL, STARTTLS, and plain
+3. Test with folders containing special characters
+4. Test with empty folders
+5. Verify EML files are valid and openable