Is it permissible to open the email and read the oath after breaking off the engagement, knowing that the oath was made with the intention of continuing the engagement and not angering the suitor who has now gone his own way?

There is no harm in reading the general section and opening mail. Expiation is required when breaking an oath, which is feeding ten poor persons or clothing them or freeing a slave. If you are unable to do that, then fast for three days, and it is more cautious to do them consecutively. Your intention does not waive the expiation according to many scholars, and it is more cautious to perform the expiation to clear oneself of responsibility.