IMAP METADATA and BURL
Mailu supports the IMAP METADATA extension and BURL message submission for compatible mail clients. METADATA lets clients store and retrieve annotations for an IMAP server or mailbox, as defined by RFC 5464.
BURL lets a submission client send a reference to message content already available from IMAP instead of downloading that content and uploading it again over SMTP. The client obtains an authorized IMAP URL with URLAUTH and submits it using BURL (RFC 4467 and RFC 4468). For authenticated submission, the front proxies the connection to the storage Dovecot service. That service resolves the URL and passes the resulting message to Postfix over the internal submission relay.
Using these extensions
No additional Mailu setup is needed. Connect to IMAP and authenticated message submission as usual. The server advertises METADATA and URLAUTH over IMAP, and advertises BURL after authentication to the submission service. A mail client must implement the relevant extensions to use them; clients without BURL support continue to submit message content over SMTP normally.
Roundcube leaves IMAP METADATA available. SnappyMail supports both METADATA and PREVIEW, and Mailu enables those capabilities in its SnappyMail defaults. Neither webmail currently implements SMTP BURL, so they continue to submit message content over SMTP normally.
Treat URLAUTH URLs as authorization-bearing references to mailbox content. Let the mail client generate and use these URLs rather than sharing or editing them manually.