If you have lots of old users which email address are expired, you have to clean the mailing list before you send your marketing emails. And also you have to clean mail list regularly to make sure all the emails in your list is validate, so you mails will have high reputation and will not be blocked. Mail reputation have impact on the SEO result of your website. So clean mailing list is very important for most websites.
Recommend the following mail cleaning service:
Email Cleaning and Validation
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