In Moodle 4.5 (possibly starting from 4.2), the Inbound Message (tool_messageinbound) task attempts to verify and create required IMAP folders. When connecting to a Dovecot IMAP server, the task repeatedly logs:
Unable to find the 'INBOX..tobeconfirmed' mailbox - creating it.

This occurs on every run, despite the mailbox folder already existing.

Cause:
The issue stems from how Moodle constructs the mailbox path in the get_confirmation_folder() method within /admin/tool/messageinbound/classes/manager.php. Moodle concatenates the personal namespace and delimiter returned by the IMAP server with the confirmation folder name.

If the IMAP server (e.g., Dovecot) returns a namespace that already includes a trailing delimiter, Moodle appends the delimiter again, resulting in a malformed mailbox path like:
INBOX..tobeconfirmed

Workaround:
Modify get_confirmation_folder() to check whether the namespace already ends with the delimiter before appending it:

if (substr($nspersonal[0], -1) === $nspersonal[1]) {

    $this->imapnamespace = $nspersonal[0];

} else {

    $this->imapnamespace = $nspersonal[0] . $nspersonal[1];

}

This prevents the double delimiter and ensures correct folder detection.

Environment:

• Moodle 4.5

• Dovecot 2.3.x IMAP server (running on Plesk w/ Ubuntu 22)

• Default IMAP namespace configuration (personal namespace: INBOX. with delimiter ".")