Email addresses on a mailing list can become outdated very quickly. Through a mailing list lifetime, it tends to become old. As a mailing list grows, the more frequent verifications it needs.
About half of email addresses become unusable in less than one year. It does not mean that mailboxes no longer exist. Users often subscribe to a newsletter, and in less than one year, lose interest in it. Sending emails to those subscribers may be costly. Some of them may choose to unsubscribe, but many will likely mark an email as spam. Often it affects the success of a marketing campaign.
Email address verification reduces risks of hard bounces and spam folder landing.
Email marketing experts suggest validating mailing lists containing less than 10 000 contacts each month. If your mailing list exceeds 10 000 email addresses, you should check it every two weeks. If your mailing list is more extensive, then you should check it before every marketing campaign.
It is a good practice to verify every new email address before adding it to a mailing list. You can use an email verification API to check email address validity before adding it. This method is less expensive. It sets a gateway and does not allow unsafe contacts into your database.