Simple Email Validation

Implement a simplified email validator. Return true if the string is a valid email address according to these rules:

1. Exactly one @ symbol — not zero, not two or more
2. Non-empty local part — the part before @ must have at least one character
3. Non-empty domain — the part after @ must have at least one character
4. Domain has at least one dot — the domain part must contain at least one '.'
5. No spaces anywhere — the entire string must be free of spaces
6. Domain does not start or end with a dot — "user@.com" and "user@com." are invalid
7. No consecutive dots in domain — "user@a..b" is invalid

This is a simplified check — it does not cover the full RFC 5322 email spec.