Collect sender addresses from incoming messages for autocomplete

Is your feature request related to a problem? Please describe.

Nextcloud Mail currently collects addresses from outgoing messages, using the To/Cc/Bcc recipients of sent mail.

This means that people who only send mail to the user, but have never received a message from the user, may not appear in Mail's address autocomplete.

For example, when using the advanced search "From" field, many valid senders are missing from the autocomplete suggestions even though the user has received many messages from them.

Describe the solution you'd like

Collect the From address of newly synchronized incoming messages, preferably for messages synchronized into the INBOX.

The existing AddressCollector and mail_coll_addresses mechanism could be reused.

The feature should also respect the existing collect-data user preference, just like outgoing address collection does.

Conceptually:

• MessageSentEvent
  • continue collecting To/Cc/Bcc addresses
• NewMessagesSynchronized
  • if mailbox is INBOX
  • collect the From addresses of the newly synchronized messages

The existing deduplication logic in AddressCollector can prevent duplicate entries.

So, if a user receives a message from:

John Example john@example.com

then john@example.com should become available in Mail address autocomplete even if the user has never sent an email to that address.

Describe alternatives you've considered

No response

Additional context

We implemented and tested this locally with Nextcloud Mail 5.12.0 by registering AddressCollectionListener for NewMessagesSynchronized and passing each incoming message's From address to the existing AddressCollector.

The feature works correctly with the existing collected-address autocomplete mechanism.

This request is primarily about collecting addresses from newly synchronized incoming messages. Historical backfilling of already-synchronized messages could be considered separately.

Nextcloud version: 34.0.2
Mailserver or service: Gmail / IMAP
Operating system: Debian GNU/Linux 13 (trixie)
PHP: 8.5.10
Web server: Apache 2.4.68
Database: PostgreSQL

Start by tracing AddressCollectionListener, AddressCollector, MessageSentEvent, and NewMessagesSynchronized, then inspect how the collect-data preference and mail_coll_addresses storage are used. Handle newly synchronized INBOX messages by collecting their From addresses while preserving existing deduplication and outgoing collection behavior. Done means newly received senders appear in autocomplete without collecting addresses when the preference is disabled.