Recipient validation fails when processing some valid recipients who exist in the directory data source but when the local part of the email address contains quotation marks.
Example: <"John Smith"@domain.com>
The full recipient email address is being used to do recipient validation, including the quote characters if the local or "user" part of the email address is quotes. Since the email address in the directory data source is unlikely to contain the quotation marks, this generally results in a failure to match the two strings.
This is a known situation and is expected behavior from an LDAP perspective. Currently there is no workaround other than to include an email alias in the directory which matches the quoted recipient address.