A formal salutation used in business letters or notices. Considered 二重敬語 (double keigo) and therefore not good Japanese; modern alternatives like 関係者各位 or 各位 are preferred.
To whom it may concern, we apologize for any inconvenience caused.
Because 関係各位殿 constitutes double keigo, it is often avoided in business documents.
A compound of 関係 (related parties), 各位 (everyone), and 殿 (honorific suffix). The combination of 各位 and 殿 creates a double honorific, which is considered stylistically incorrect in modern Japanese.