IMAP APPEND command fails to parse valid RFC 3501 syntax

[syrenius]

First of all, thanks for this fantastic project!
In one of my applications we create email drafts and use IMAP APPEND. It works well with the production servers.
However, using mokapi, IMAP APPEND command fails with a parse error when the message includes both flags and an internal date, even though the command is syntactically valid per RFC 3501 §6.3.11.

To Reproduce:

Connect to Mokapi's IMAP server using Python's imaplib and call append with flags and a date:

import imaplib, time

server = imaplib.IMAP4(host="localhost", port=143)
server.login("user@example.com", "test")
server.select("Drafts")
server.append(
    '"Drafts"',
    r"\Draft",
    imaplib.Time2Internaldate(time.time()),
    b"Subject: Test\r\n\r\nBody"
)

To rule out an issue with imaplib I also did a similar test using Thunderbird, which leads to the same issue.

Expected behavior

The message is appended to the Drafts folder.

Actual behavior

Mokapi logs the following error:

imap: parse command failed 'LFKG3 APPEND "Drafts" (\Draft) "26-Apr-2026 14:20:31 +0200" {53367}': expected SP, got "Drafts" (\Draft) "26-Apr-2026 14:20:31 +0200" {53367}

The same error occurs with an unquoted mailbox name (Drafts instead of "Drafts").

Mokapi configuration

mail: '1.0'
info:
  title: Dev Mail Server
servers:
  imap:
    host: :143
    protocol: imap
mailboxes:
  user@example.com:
    username: user@example.com
    password: "test"
    folders:
      Drafts:
        flags: [\\Drafts]
settings:
  allowPlainTextLogin: true

RFC 3501 reference

The APPEND syntax per RFC 3501:

append = "APPEND" SP mailbox 1*append-message
append-message = [SP flag-list] [SP date-time] SP literal

Both flags and date-time are optional but valid. The command Mokapi rejects is fully conformant.

Environment

Mokapi version: latest (Docker image mokapi/mokapi:latest)
Client: Python imaplib (Python 3.11)