Invalid contacts can be categorized into several types: - Hard Bounces: These are permanent delivery failures due to reasons like non-existent email addresses. - Soft Bounces: These are temporary issues such as a full inbox or server problems. - Spam Traps: These are email addresses set up to catch spammers and can severely damage your reputation if you send emails to them. - Typo Errors: These are email addresses with common misspellings or syntax errors.