Please help, other people's mailbox in my yard

My neighbor is an apartment building. The owner is a realtor, he does not live there. The building is at a corner, does not have much frontage, so he put a row of mailboxes in my yard. I talked with him once asking him to move them. He said, "This is not your property. Your property starts 15 ft from the curb. This is city's easement, you just have the responsibility to maintain it. It is like the street, you do not own the street, anyone can park a car on the street in front of your house." He is a realtor, what he said is probably true, but, what can I do to get those ugly mailboxes moved away? Please help.