I just found out I’m making less than everyone else in my department, even though I’ve been here longer than some. How can I bring this up in a written email?

As the title says, I just found out I’m the lowest paid employee in my department. We do the same job so it’s not like some are higher level or anything like that. In fact I’ve been here longer than some of the other employees. Everyone got an increase last July and I thought we all got the same amount but it turns out that wasn’t the case. Is there anything I can do. If it helps I’m in California. Also, if anyone has a way I can go about this without risking retaliation please let me know.