If you see a yellow warning like the one pictured below, it means that we're having problems delivering to the email address you provided. If our emails to you continue to bounce back as undeliverable, we'll stop attempting delivery for some of your notifications.
Based on the account notices preferences you've selected, Twitter may email you notifications about account activities like new messages, new followers, or Twitter newsletters. But, if we've stopped attempting delivery for these emails, you may also be unable to receive important emails, like password reset emails.