BUG: Dont use assert for a test

When running with -O or -OO options `assert` are dismissed, so the line which call regular expression to test the email is not executed.

In place of 
```python3
assert re.match(VALID_ADDRESS_REGEXP, email) is not None
```

You should write
```python3
if re.match(VALID_ADDRESS_REGEXP, email) is None:
    return False
```
(or raise an exception if you prefer)

A+
L.Pointal