Add dual authentication support and comprehensive provider examples

Core functionality:
- Add dual authentication support (username + email fallback)
- Enhance IMAPConnection to try username first, then email
- Add SOURCE_USERNAME and DEST_USERNAME configuration options
- Improve authentication error handling and logging

Configuration updates:
- Add Host Europe to Securehost.de migration examples
- Include authentication method explanations for various providers
- Add comprehensive provider-specific settings (Host Europe, Securehost.de, etc.)
- Document username vs email login methods with examples

Documentation updates:
- Add dual authentication section in README
- Include Host Europe and Securehost.de specific examples
- Expand troubleshooting section with authentication help
- Add provider-specific troubleshooting guidance
- Include migration best practices

Features:
- Automatic fallback from username to email authentication
- Enhanced logging showing which authentication method succeeded
- Support for various hosting providers and their login requirements
- Improved error messages for authentication failures

email_migration.py

@@ -30,10 +30,11 @@ def setup_logging(log_level):
     return logging.getLogger(__name__)
 
 class IMAPConnection:
-    def __init__(self, server, port, email_addr, password, use_ssl=True, timeout=60):
+    def __init__(self, server, port, email_addr, username, password, use_ssl=True, timeout=60):
         self.server = server
         self.port = port
         self.email = email_addr
+        self.username = username
         self.password = password
         self.use_ssl = use_ssl
         self.timeout = timeout
@@ -49,8 +50,34 @@ class IMAPConnection:
                 self.connection = imaplib.IMAP4(self.server, self.port)
 
             self.connection.sock.settimeout(self.timeout)
-            self.connection.login(self.email, self.password)
-            self.logger.info(f"Connected to {self.server} as {self.email}")
+
+            # Try authentication with username first (if provided), then fall back to email
+            login_success = False
+            login_user = None
+
+            if self.username and self.username.strip():
+                try:
+                    self.connection.login(self.username, self.password)
+                    login_user = self.username
+                    login_success = True
+                    self.logger.info(f"Connected to {self.server} using username: {self.username}")
+                except Exception as username_error:
+                    self.logger.debug(f"Username login failed for {self.username}: {username_error}")
+
+            # If username login failed or no username provided, try email
+            if not login_success:
+                try:
+                    self.connection.login(self.email, self.password)
+                    login_user = self.email
+                    login_success = True
+                    self.logger.info(f"Connected to {self.server} using email: {self.email}")
+                except Exception as email_error:
+                    self.logger.error(f"Email login failed for {self.email}: {email_error}")
+
+            if not login_success:
+                self.logger.error(f"Failed to authenticate with {self.server} using both username and email")
+                return False
+
             return True
         except Exception as e:
             self.logger.error(f"Failed to connect to {self.server}: {e}")
@@ -144,7 +171,10 @@ class EmailMigrator:
         if not self.import_folder_name:
             self.import_folder_name = None
 
-        self.logger.info(f"Import folder configuration: {'All emails will be imported to subfolders within \"' + self.import_folder_name + '\"' if self.import_folder_name else 'All emails will be imported directly to INBOX'}")
+        if self.import_folder_name:
+            self.logger.info(f"Import folder configuration: All emails will be imported to subfolders within \"{self.import_folder_name}\"")
+        else:
+            self.logger.info("Import folder configuration: All emails will be imported directly to INBOX")
 
         include_str = config.get('INCLUDE_FOLDERS', '')
         exclude_str = config.get('EXCLUDE_FOLDERS', '')
@@ -158,6 +188,7 @@ class EmailMigrator:
             config['SOURCE_IMAP_SERVER'],
             int(config['SOURCE_IMAP_PORT']),
             config['SOURCE_EMAIL'],
+            config.get('SOURCE_USERNAME', ''),
             config['SOURCE_PASSWORD'],
             config.get('SOURCE_IMAP_USE_SSL', 'True').lower() == 'true',
             timeout
@@ -167,6 +198,7 @@ class EmailMigrator:
             config['DEST_IMAP_SERVER'],
             int(config['DEST_IMAP_PORT']),
             config['DEST_EMAIL'],
+            config.get('DEST_USERNAME', ''),
             config['DEST_PASSWORD'],
             config.get('DEST_IMAP_USE_SSL', 'True').lower() == 'true',
             timeout