We can mark the account as valid and attempt to send to this user. However, the user may be marked invalid in the future if the email address returns an undeliverable hard bounce.
  1. Select Email > Invalid accounts.
  2. Search for the email address in question and click the pencil icon to edit the account.
  3. Click Mark as valid at the top of the account editor.
Note: you can only remove one Invalid Account at a time. 

For more information on Invalid Accounts see our documentation here.

Per the documentation:
In Invalid accounts, you can view email address accounts that have been reported as invalid by their hosting Internet Service Provider (ISP). For example, an ISP may report an email address account as invalid due to a nonexistent or canceled account. Typical causes of an invalid account occur when the email address contains a typographical error or the email account is canceled due to inactivity. Accounts that bounce email for another reason, such as a temporary network issue, a full mailbox, or a content filter, do not appear in Invalid accounts.

To prevent your email address from being blacklisted, the program does not send email to invalid accounts.

Do not remove an email address unless you've confirmed with the recipient that it is truly valid.