When trying to send email to a Dovecot user, you get the following message:
This is the mail system at host hostname.localdomain. I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below. For further assistance, please send mail to postmaster. If you do so, please include this problem report. You can delete your own text from the attached returned message. The mail system <[email protected]> (expanded from <[email protected]>): can't create user output file
The problem here is the user’s mailbox is full. If you’re using Virtualmin you might have hit the default 50MB mailbox size parameter. Increase the size of the mailbox and try again.