A kind message can be plain enough to believe on the first reading.

Kindness often arrives overdressed. Wanting it to sound sincere, people layer on qualifications and careful phrasing until the reader has to work out what is being said, and warmth that needs decoding does not land warmly. A plain sentence travels further because there is nothing in it to doubt. Simple praise is easier to believe than ornate praise.

Write the kind message you have been meaning to send, then cut it in half. Name the specific thing they did and what it meant to you, and leave it at that.