client with whom we have had business relations for many years 意味

• 当社の永年の取引先
  

• as we have not had the pleasure of doing business with you before:    御社｛おんしゃ｝とはこれまでに取引をしたことがありませんので
• in the business for many years:    《be ～》長年この仕事｛しごと｝に従事｛じゅうじ｝している
• in the business many years:    《be ～》長年この仕事｛しごと｝に従事｛じゅうじ｝している