Some years ago a postman in Spain was caught with a sack in his home with 3000 votes that never were delivered to their destiny. This happened many months after the election. Last year in December some thieves broke into a small village post office and stole the money and the sack with 124 mail votes that were found later partially burnt. The few voters involved were allowed to re-send his vote before the elections and an organization of 12 people were detained later for this an other assaults to post offices before the local elections.

So yes, mail voting has a few issues still, not a lot, but is not impossible to break. If every worker in the post office is from one party and nobody in that party votes by mail, bribing and corrupting the system is doable, and even easy; specially if your work depends on it.

If you do this in small populations or at scale you could easily remove enough votes from the system until winning the place by one single vote. If you are malignant enough you could even blame later millions of "lazy" people for not voting and laugh about it.

Mail voting depends on trusting what your postman is not a plant.