One of the best ways to improve email deliverability is to build a solid reputation with email services by sending high-quality emails day after day.
For example, you shouldn’t just send an email to 500,000 people in one day (that’s a huge red flag to servers). Instead, build up to a large email send by breaking it up over several days, and increasing the volume with each send (in other words: sending to 100,000 people five days in a row isn't great either).
P.S. If you're a Splash customer and considering a large-volume email send, talk to your CSM about the best approach for your strategy.