Which parts of the brain are used when identifying and sorting email messages? When most people view their email inboxes, they'll see several different types of messages; for example, emails which have been auto-generated by services we use, communications with real humans which are time sensitive, communications which aren't time sensitive, etc. 

Which parts of the brain are used when viewing, identifying, and categorizing these messages? Does this activity fall into a specific category of mental activity?
The insular cortex seems to play a role in sorting based on salience - how important something is to us (Uddin, 2015). In addition, the anterior cingulate cortex (ACC), commonly associated with error detection, is relevant. Some parts of the insular cortex have also been linked to a sense of self, so this seems reasonable.

The mental activities involved include: detection (What?), attention (Where?), categorical learning (How?), and planning (Which - when?). There are thousands of studies on visual search, visual attention, stimulus detection, salience processing, and categorical learning. Categorical learning implies that one can improve in the process of categorization, but attention is a prerequisite for learning to occur, so multiple brain areas are involved.

Reference

Uddin, L. Q. (2015). Salience processing and insular cortical function and dysfunction. Nature Reviews Neuroscience, 16, 55–61. doi:10.1038/nrn3857

http://www.nature.com/nrn/journal/v16/n1/abs/nrn3857.html