I'm running Dovecot on my mail server and storing messages in the Maildir++ format. I have the following directories (as well as the usual Maildir directories) in /var/mail/user:
.Something.2013
.Something.2013.Foo
.Something.2013.Bar
.Something.2014.Foo
.Something.2014.Bar
When I view these directories in a mail client (Roundcube WebMail, Apple Mail) the Something/2014 folder has a slightly different icon and doesn't allow mail to be put into it, presumably because it doesn't actually exist as a mail directory.
Oddly enough, none of the above directories are listed in the subscriptions file; just Something is.
Can I just create this folder with mkdir .Something.2014 or is there more I need to do?
Yes, you can simply create the missing .Something.2014 folder. It looks different because having that missing folder is valid--it's a type of folder that may only contain child folders, not messages (the child folders can contain messages, though). Most of the time, this is simply an error due to not creating the complete folder chain (in this case, not creating .Something.2014) or accidental use of a "." in a folder name. You can see this if you create a folder in Thunderbird with a "." in the name.