If you are writing a formal letter to a government department or business, the closing atentamente is the closest thing to saying "sincerely" in English, and the most commonly used. You also can say "le saluda atentamente," which is more like "sincerely yours." If the letter is addressed to more than one person, write this "les saluda atentamente." The closing "cordialmente" typically is used after you've established a more familiar relationship with the person to whom you're writing. It's appropriate for repeated correspondence when you have a regular business or collegial relationship with someone. Closings for Spanish correspondence typically are more affectionate than those used in English, if you think about the direct translation of the words. There are other warm closings for personal letters, such as "un abrazo" (a hug), "cariñosos saludos" (similar to "kind regards"), or "afectuosamente" (affectionately). The Spanish phrase "besos y abrazos" literally means "kisses and hugs." As you can probably imagine, this closing is more appropriate if you're writing to someone you would actually hug or kiss on a regular basis. Some other closings you can use with family and close friends include "con todo mi cariño" (with all my caring) or "con todo mi afecto" (with all my affection).

TL;DR in Indonesian:
Gunakan kata atentamente untuk mengakhiri sebuah korespondensi bisnis. Gunakan kata cordialmente untuk mengakhiri korespondensi bisnis yang lebih kasual. Gunakan ucapan yang bermakna lebih intim di dalam surat yang bersifat personal. Gunakan frasa besos y abrazos untuk mengakhiri surat yang dikirimkan kepada kerabat dan sahabat terdekat Anda.