When starting an email message, do you write a small letter or a big one after "Good afternoon," (comma is important)
I personally believe that once there is a comma, it is considered as a standard sentence and therefore a small letter should be used. This is how it works in my native language.
Is it the same way in English language?
Would you please share with me some link where is this confirmed?
You use the comma if you wish to address the person's name eg. Good afternoon, Mr Brown. If you wish to continue with the sentence, you use the word 'and' eg. 'Good afternoon and welcome to .....'"Good afternoon,...."
If not, I think it is better to just write ' Good afternoon.' and start a new sentence.
not a teacher
For letter writing, even email writing, the rule of thumb is as follows. If the salutation and the opening sentence are on the same line, then use lowercase to begin the opening sentence; if the salutation and the opening sentence are on different lines, then use a capital to begin the opening sentence:
Line 1: Good morning, how has your day been so far?________________
Line 1: Good morning(,)
Line 2: How has your day been so far?
Note that, it is now increasingly common to omit the comma after the salutation. (open vs closed punctuation)