Dovecot creates folder, Postfix expects file
Solution 1:
The answer is extremely simple. The path in /etc/postfix/vmail_mailbox is missing a trailing slash. When there is no trailing slash, postfix will treat it as a file and thereby assume the mailbox is in Mailbox (and not Maildir) format.
By appending the trailing slash, postfix will correctly consider the directory to be of maildir format and treat it accordingly.
Example:
[email protected] domain.tld/user/
[email protected] domain.tld/user2/