That is, even if I know they have the evening – I write ‘good morning’ if we have the morning. In a sense, I just tell you that we have a good morning.

Here I know what is happening there in your educated circles, but if a person says good morning, he does not report that he, such a cool, morning was good, but wishes a good morning to the interlocutor.
And the interlocutor can read the letter in the morning, in the day and at night. Even in the evening.