Yes, it is a legitimate business expense.
Specifically it is a marketing expense. For example, in my business, I have hired a firm to help me distribute a press release on a new project. They are good at doing this and getting publications to publish the release or review the topic of the press release. But all of that does not directly result in any profit to me, it only results in exposure. Press releases do not "sell" anything or offer to sell anything, the hope is they interest people to contact my firm, and perhaps buy, but really it is just publicity.
Since at this writing it is just after Christmas, I will also use the example of a business donating money to buy Christmas gifts for the kids in Child Protective Service; which does not provide gifts to kids at Christmas. The many that have do not sell a thing, but they are shown on the news, presenting big checks, and that is a business expense too: It generates good will for their restaurant or bank or used car lot, it gets their name out there, and maybe people that could be new customers will think of them kindly. The same is true for sponsoring shows on the Public Broadcasting channel, or offering prizes for a school raffle, etc. There is no direct income from any of those things, it is just exposure and perhaps recognition, precisely what you are getting by submitting to an unpaid publication.
Your postage is a marketing expense intended to get your work and name as an author greater exposure in order to build your reputation as an author with verifiable credits. It is building good will and public awareness.