At Mailshake, email marketers can't guarantee that their emails will reach their subscribers' inboxes or be opened, but they can take steps to maximize deliverability by warming up their IP addresses, implementing double opt-in confirmations, making the opt-out process easier, and setting subscriber expectations. Factors such as low sender scores, spammy content, high bounce rates, negative recipient actions, and clone phishing attacks can decrease email deliverability, while segmentation and cleaning the email list are also crucial for improving deliverability and engagement rates.