List hygiene plays a role in the delivery race. It is important to maintain a clean mailing list and remove bounced/undeliverable emails because a lot of ISP mail servers have been known to block a sender’s email domain for repeated sending messages to email addresses from a non-existing domain.
Most spammers don’t have actual email addresses. Spammers are known to use a software that takes random email addresses and they do not care about bounced or undelivered emails. Hence, ISP assumes that the sender is trying to spam and may block or blacklist his email domain. It will be quite problematic to resolve the matter (block/blacklist) and the only thing to do is to register a new email domain or use another email domain to send emails out.