Does concating a string like this open me up to SQL injection?
No. That value will always be interpreted as a string and there's no way to put anything into the emaildomain column that could make the query execute unexpected code. It's either going to find matches or it's not.

Of course, this is relevant to the specific code sample you posted, and only that. You should always be concerned about any data you accept from a user... since you might not know where else it might be used now or in the future.