This appears to be a lock file that has been left over. Potentially from a file that is still open or did not close properly or something.
The file shows up in the customer’s Maildir and is hidden so it starts with a . and in the Maildir all IMAP folders also start with a . so this file shows up as an IMAP folder in the e-mail clients.
Customers cannot delete this leftover file through any means. Simply put in a ticket if they want one of these deleted. Hay will have an administrator manually remove the file.
I find it happens to certain customers repeatedly so it must have to do with their software, workflow, or something.
Here’s the stuff I found here online:
D2. What is a “silly rename”? Why do these .nfsXXXXX files keep showing up?
A. Unix applications often open a scratch file and then unlink it. They do this so that the file is not visible in the file system name space to any other applications, and so that the system will automatically clean up (delete) the file when the application exits. This is known as “delete on last close”, and is a tradition among Unix applications.
Because of the design of the NFS protocol, there is no way for a file to be deleted from the name space but still remain in use by an application. Thus NFS clients have to emulate this using what already exists in the protocol. If an open file is unlinked, an NFS client renames it to a special name that looks like “.nfsXXXXX”. This “hides” the file while it remains in use. This is known as a “silly rename.” Note that NFS servers have nothing to do with this behavior.
After all applications on a client have closed the silly-renamed file, the client automatically finishes the unlink by deleting the file on the server. Generally this is effective, but if the client crashes before the file is removed, it will leave the .nfsXXXXX file. If you are sure that the applications using these files are no longer running, it is safe to delete these files manually.
The NFS version 4 protocol is stateful, and could actually support delete-on-last-close. Unfortunately there isn’t an easy way to do this and remain backwards-compatible with version 2 and 3 accessors.