a note of thanks; grateful letter; letter of gratitude (Ramzess); thank-you note (Дмитрий_Р); herogram (a message of congratulation sent, e.g., by a school to pupil's parents, or by a boss to an employee for a well-executed piece of work. Good job, well done are the words that are often used in such circumstances Anglophile); bread-and-butter letter (Andrey Truhachev); thank-you letter; thank you letter (Andy)

thank you note (A note or email message sent to a person to thank them for something received, or for a meeting)

благодарственное письмо

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