Why is IS NOT NULL false when checking a row type?

<row-type> IS NOT NULL

As @Pavel provided, the check <row-type> IS NOT NULL doesn't work like you seem to expect. It returns TRUE if, and only if, every single column is NOT NULL.

You could invert your test expression:

IF rowt IS NULL THEN
   -- do nothing
ELSE 
   RAISE EXCEPTION 'email address, %, already registered.' , eml;
END IF;

Any row you find contains at least one column that is NOT NULL, therefore rowt IS NULL only returns TRUE if nothing is found.

See:

• IS NOT NULL test for a record does not return TRUE when variable is set
• NOT NULL constraint over a set of columns

Leaves a corner case for tables that allow all-NULL rows, though.

Better solutions

Test the special variable FOUND instead (like @Mike commented):

PERFORM FROM email WHERE email_email = eml;

IF FOUND THEN
   RAISE EXCEPTION 'email, %, already registered.', eml;
END IF;

Since we are not actually interested in the returned row, replace SELECT with PERFORM to discard the result. Either sets the special variable FOUND accordingly.
And the SELECT list (or PERFORM list, really) can be empty as only the existence of a row matters.

Simpler, yet, use EXISTS:

IF EXISTS (SELECT FROM email WHERE email_email = eml) THEN
   RAISE EXCEPTION 'email, %, already registered.', eml;
END IF;

See:

• PL/pgSQL checking if a row exists

Test on NULL for ROW type is specific:

postgres=# SELECT r, r IS NULL AS "is null", r IS NOT NULL AS "is not null" 
              FROM (VALUES(NULL,NULL),
                          (10, NULL), 
                          (10,10)) r ;
    r    | is null  | is not null 
---------+----------+--------------
 (,)     | t        | f
 (10,)   | f        | f
 (10,10) | f        | t
 (3 rows)

So NOT NULL returns true only when all fields are not null.