My bike has grooves in the seatpost as shown in the picture. The grooves do stop slightly further down the post, but the best saddle height for me is when the grooves partially go down into the seat tube. To me this looks as if water/rain can just flow down the grooves into the (steel) seat tube. What are the dangers of steel frames rusting from the inside out if water gets in? Is this seatpost designed to be used at a minimum height so that the grooves are completely above the clamp? (I can't see any minimum insertion height marked on the post.)
The best thing to do is apply a thin layer of grease to the inside of the seat tube and the outside of the post (only the part of the post inside the seat tube needs grease) and set the seat height according to what fits you.
The layer of grease will prevent the odd drip from causing rust.
If the bike is stored in a dry place after riding in the rain it will be able to dry out.
The bikes with seat posts rusted in that I've seen were the ones that were stored outside and never cared for.
Another option is to cut a longer length of old inner tube, and put it over the seat post before mounting to bike. Then once the height is right, lower the "skirt" over the top of the clamp.
Its not perfect on your bike because there's little "throat" above the clamp, so for you there will be minimal overlap. Sorry.
Aside - your bike has a pinch slot for the seatpost clamp, which is another open area, perfectly aligned to catch road water off the back wheel. So don't forget that ingress point.
Lastly, make sure your Bottom Bracket has a clear drain hole - water does more damage when it sits, so if you can let it fall out the bottom that will reduce problems with standing water inside the frame. Storing the bike inside helps, as does keeping it away from cold.
Much sooner than any corosion you will have problems from sand or other mud particles getting between the seatpost and the seattube and causing horrible clicking with every pedal stroke or when shifting your weight on the saddle.
You can try applying the trick I employ, having myself big proplems with that even without any grooves. So far I used electrical tape to seal what I can but this sealing instruction appears to be more robust https://www.bikeradar.com/features/budget-bike-tech-seal-your-frame-from-mud-and-water/ (uses silicone and a rubber o-ring).