After assigning a Topic (for example, within the inbox), if you find that the Topic is not sticking or if it disappears after moving to another folder, it could be that your mail server does not support the IMAP Keyword Standard, which is an internet standard that Postbox requires for Topics to work.
We find this most often with Kerio servers and some AIM servers, but there could be others.
An example of an IMAP server that supports the Keyword standard is Gmail. To test whether your Postbox installation is working with Topics, please set up a Gmail account and assign a few topics to your messages.