So what's your end game, here?
He is, almost definitely, having another affair. He has had previous affairs. This has been an issue for four years, and appears to be worsening. He also has a porn problem, and continues to watch it despite knowing that it upsets you.
And, he knows that you check his phone, and he's using it to have affairs anyway, because he knows that you won't leave. So you can be very sure that he won't stop, regardless of what you say or do. He doesn't care that you know. He deleted the message to avoid an argument because it'd give him an easier life.
If you don't want to leave, I can't see this getting any better. You are probably better trying to accept it and stopping yourself from checking his phone, because you can be certain that if you don't find anything, he's deleted it, and if you do, it's probably the tip of the iceberg. You'll drive yourself insane checking.
If you do want to leave, there are wise people here who can help you make a plan.
He won't change, though. He doesn't want to, and you can't make him.