When you are replying as an agent to email notifications from your mailbox, there could be instances where you would also want to modify the ticket properties. However, as you are working from your mailbox, this would not be directly possible.
However, use the Email Commands feature in Freshdesk, agents can reply from their mailbox and also modify the ticket properties. This is done using a delimiter and including the fields followed by the value which would have to be added to those fields.
An example is, @Simonsays "status":"pending", "priority":"medium", "agent":"John Robert" @Simonsays, where, @Simonsays is the delimiter, within which the Ticket Properties and values to which they would have to be updated are mentioned.
Here's an article detailed instructions on the working of email commands.