expression
A formal salutation used at the beginning of a letter or document when the recipient is unknown or unspecified. Common in business or official correspondence.
関係当事者殿、このたびはお世話になっております。
To whom it may concern, thank you for your continued support.
A set phrase composed of 関係 (relation), 当事者 (concerned party), and the honorific suffix 殿 (Mr./Ms.).