Trigger or rule to convert all strings to lower case
Because we're talking about email addresses you should first read my answer to the question, What is the best way to store an email address in PostgreSQL?. I would adopt that method. It's not too late.

That said, the code above is certainly not ideal. You should instead just migrate to citext. This overrides the = to essentially be lower(a) = lower(b)

ALTER TABLE your_table
  ALTER COLUMN the_column
  SET DATA TYPE citext;