When giving us context, you mention that the message was received a month ago. That's good. Include such temporal information in the sentence you're trying to construct, and allow those time phrases to guide your choice of tense. Do not rely only upon the tense to convey the temporal information.
I have just now seen your message, which you sent a month ago.
The phrase a month ago establishes a discrete point-in-time in the past. Thus, a simple past is suitable.
Just now means very recently, a few moments ago. You're referring to a very recent event that colors your present circumstances. Thus, the present-perfect is suitable.
Let's imagine that you are a procrastinator.
Last week I saw the message which you had sent three weeks earlier. I'm sorry
it has taken me so long to reply.