New answers tagged politeness
I think that it is perfectly fine to use -さん when writing the letter, since that's what you normally use anyway. If you often speak to him informally, then you can revert to that style of speech after the opening. I find this to be a good example: 例えば・・○○さん、お誕生日おめでとうございます。の後に また今度職場の皆でカラオケに行きましょう！○○さんのあまい歌声をまた聞かせてくださいね。 ...
The explanation by user3688956 gives the right interpretation. The one with お is a little more polite and more respectful, though it can be taken as slightly more distanced, if used for very closed friends. I'll illustrate the nuance. For example, compare calling your father "dad" or "father". While the use of the word father doesn't warrant a lack of ...
お誕生日おめでとう is a little bit more polite than 誕生日おめでとう. the point is a little bit..
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