There are 3 different ways to specify categories in your email and they all involve the subject line.
Note the category must already be set up, Postie will not create new categories.
(although there is an AddOn that will do this now – Create Categories)
The first way is to put a category name (or partial name) or a category id then a colon (:) then the actual subject.
Usage (in the subject line):
<category name>: Real subject <category id>: Real subject <partial category name>: Real subject
The second and third ways are very similar and allow you to specify multiple categories. Enclose the category name (or partial name) or id in square brackets () or between dashes (-).
[category1]  Real subject    Real subject [cat] Real subject -category1- -19- Real subject -1- -2- -10- Real subject -cat- Real subject
If you don’t want to make use of partial matches you can turn off the “Match short category” setting.
This technique also works with custom taxonomies. You may have to enable the “Force User Login” setting depending on how the taxonomies were set up.
Specifying Post Type/Post Format
If you are trying to also specify the post type/format you need to do that first, like so:
myposttype//[mycategory] my subject
Disabling Category/Taxonomy Matching
Set all of the following settings to “No”