Hello @OTAS ,
You can try re-adding the contact's email address as a single contact, or as part of a type/past multiple or file upload import. You may be able to look through his engagement activity to see if he had bounced or unsubscribed, which may indicate the why he may have been deleted at some point by the account owner or a manager. Beyond that we wouldn't really have any further insight, especially if it's been several months or years since the deletion.
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