[RightDeve]

That's the double-edged sword for SP though.
On one hand, it allows you to move your units in a flexible manner, so you get the feel of strings of "cause and effect".
On the other hand, it allows for this rather gamey tactic.

If it's annoying to the human opponent and potentially destroys any enjoyment of the game, he should stop doing such tactic (such tactic rarely happens in real life anyway). PBEM is all about mutual enjoyment instead of mere winning through all means.

That being said, taking cover behind dust is not entirely gamey, especially if done non-systematically & non-intentionally. For example in a chaos of battle, after suffering some damage/casualty, such tanks may indeed retreat back behind the dust screen it kicked up previously (works in real life too).

So if you're annoyed by such behavior, tell him to stop.
Or, to make things work within the game's mechanics: Set your guns' range to something that allows for guaranteed hits (or even use OpFire filter). So when the tanks advance (obviously while still ahead of their dust), they'll take hits from the guns' Op-Fire. Hopefully with shorter ranges you'll have more luck in destroying them. With Op-Fire Filter, you can even filter out those pesky little jeeps & carriers, and only Op-Fire the juicy tanks.

Good luck!