Assuming that you have a unique index on table1.email, then why not use this more simple command:
```
INSERT OR IGNORE INTO table1(email) VALUES ('test@domain.com');
```
Of course, if you either have no useable index then you can use a longer more inefficient method such as:
```
INSERT INTO table1 (email)
SELECT email
FROM (
SELECT 'test@domain.com' AS email
) AS o
WHERE NOT EXISTS (
SELECT *
FROM table1
WHERE email == o.email
);
```
The former is much shorter and staightforward while the latter is much longer and slower, but is standard SQL that should work unchanged on all SQL platforms since about 1985.