[choilund]

Summary

Fixes KeyError: 'multipart/related' when parsing incoming emails that contain rich HTML with inline images or nested multipart structures (such as notifications sent by Vaultwarden, Apple Mail, Outlook, and NVR software).

Resolves #135
Relates to #9, #136

Problem

In Python's email.message.EmailMessage.get_body(), the default preference list is:

def get_body(self, preferencelist=('related', 'html', 'plain')):

When an incoming email contains an HTML body with inline images or attachments, the MIME structure wraps it in a multipart/related container. Because 'related' is the first preference, msg.get_body() returns the multipart container envelope itself rather than the leaf body part.

Attempting to read content from this container raises KeyError: 'multipart/related' because standard library content managers only extract content from leaf MIME parts. Furthermore, when multipart/related is nested inside multipart/alternative, the default preference causes get_body() to bypass alternatives (issue #135).

Solution

Pass preferencelist=('html', 'plain') to msg.get_body() in src/mailrise/smtp.py:

py_body_part = msg.get_body(preferencelist=('html', 'plain'))

• Standard Library Traversal: Directly instructs Python's MIME parser to traverse multipart/related containers and return the actual leaf text payload (text/html or text/plain).
• 100% Backwards Compatible: Retains Mailrise's default behavior of preferring HTML when present and falling back to plain text, preserving existing contracts and passing all existing unit tests in tests/test_smtp.py.
• Safe Fallback: Retains the existing _getmultiparttext helper as an additional fallback for non-standard MIME hierarchies.
• No Breaking Config Shifts: Unlike PR Prefer plain text body when available #136, this does not flip default behavior to plain text or alter config schemas, making it a safe, drop-in bugfix.

Testing

• Added test_multipart_nested_related() to tests/test_smtp.py covering an email with nested multipart/alternative containing text/plain and a multipart/related HTML part with inline image attachments.
• Verified that all assertions in tests/test_smtp.py pass.