More precisely : I work in a company that doesn't have any design culture. Some people tends to think a little more in a user-centered way but not the important one. Product Manager only cares about the development time, CEO only care about business strategy against competitors, and in the end we are never able to put in front the expected features (asked by users) above the stategic & market oriented features (defined by CEO/PM).
Let's take an example : few months ago I've been reported that users want to see immediately when a message is read or unread. The proposed solution was : "we have to display in bold the name of the message in the list, and when you read it it turns back to regular, but just for you". But then the Lead Developper said that would need a lot of time to do, because we don't know actually when a message have been read and by who. Since this moment, the Product Manager just delayed the feature, again and again, saying that the impact was too low as compared as the time needed to do it. Ultimately, it even became : "we'll probably never do it at all".
So, as a UX designer, how am I supposed to defend users when these people are taking decisions and just look at me like what I'm saying is useless ?
While this is probably a losing battle and nothing you do will change the outcome, I think you should consider having a conversation with your PM and ask what are the scope of your responsibilities and your expectations regarding your work. Obviously, all in a very respectful way and explaining that your question is to optimize the time of the company and not do things that may not be necessary.
That way, he will be forced to give you a concrete answer. If he tells you that your responsibility is to generate the best possible user experience, then you can document what you want to do, explaining the pros and cons of your decision, and how this change will result in an improvement of the experience of those users. It is very important that you consider your company's competition: if the competition does not do it, you can "sell" it as an exclusive and differential feature.
Now, the user case that you mention seems pretty trivial and in fact it is default in almost any application of messages, so either there is a lack of data in your description, or the problem is another. And unfortunately, if the problem is interpersonal relationships, egos or jealousy, I doubt very much that you can do anything.
Document everything and do your best to explain the cost-benefit differential, talk to your PM, set the scope of your responsibilities. You really can't do much more than this, but this is the least you should do.