Teemo's E doesn't proc rylias or corrupting potion

it's not a spell, so any spell activated items will not work.

Teemo's E would be too op if it procced spell effects since it is a permanent on hit effect to his AAs. Coupling it with Rylai's for example, would give Teemo half of Ashe's passive (aa movespeed slow).

Intended, try building frozen mallet tho

It's intended.

I guess you'll just have to settle for his autoattacks proccing it.

intended. It's an on-hit effect (like Wit's End, Bork's passive, or Devourer's), not an on-ability effect (like Rylai or Liandry's)

Would be ridiculous if it procced a Mage effect on every auto attack (both Rylai and Liandry is already annoying as is on Teemo's shrooms, and sometimes Q)