Having a working DNS cofiguation is generally a prerequisite to sending email.
There is of course email smtpd software that allows receiving email using only IP addresses. Email predates DNS. Two people running their own smptd's could exchange email directly using only IP addresses, without the need for DNS. But the way most people use email today, delegating total control over it to third parties, email is highly dependent on DNS.
There is of course email smtpd software that allows receiving email using only IP addresses. Email predates DNS. Two people running their own smptd's could exchange email directly using only IP addresses, without the need for DNS. But the way most people use email today, delegating total control over it to third parties, email is highly dependent on DNS.