Ta'anit Esther for a Mailman

Q: Is a mailman who is walking all day in the sun exempt from fasting on Ta'anit Esther since there is a risk of dehydration? What about the other fasts?
A: He is obligated to fast on Ta'anit Esther. Although it is not as strict as the other fasts and the Halachah therefore says that there is a leniency on Ta'anit Esther that someone who has eye pains is exempt, all others who are healthy should not separate from the community (Rama, Orach Chaim 686:2). The Magen Avraham (#4) comments that even someone who is traveling and it is difficult for him to fast is obligated to do so. A mailman is similar to someone who is traveling. He is therefore obligated in all of the fasts. Obviously, if during the fast he feels in danger of dehydration, he has to drink a lot of water. But if he knows from the outset that if he works on a fast day he will need to drink, this is as if he made a condition that he would not fast. He cannot enter this situation and he needs to work it out such as switching shifts, taking a day off or distributing the mail slowly.