Saying "hi" in the subject of an email is probably the worst mistake that one can make in academic emails. I used to do this in my undergraduate years, so i thought it would be good to share the following emails.
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From: me
To: Mr. B
Subject: hi

good evening,

i have seen the 2nd homework results updated.
however there is a rumor that the percentage of
the homework is 0. and also on the sllyabus at STARS
there is no homework percentage.

are you going to add the homework points to our final grade?

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From: Mr. B
To: me
Subject: Re: hi

Which course is this? Who are you?

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From: me
To: Mr. B
Subject: Re: hi

:)
algebra 1

NOTE : [ Sender's name appears in the inbox, so i supposed i didn't need to answer 'Who are you?'. ]

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From: Mr. B
To: me
Subject: Re: hi

I still do not know who you are. In e-mail, as with written letters
and the telephone, it is normal to give your name. Okay, it does
not matter. (You are obviously not familiar with the medium. A
subject such as ``homework credit'' might have been more to
the point than ``hi''.)
To answer the question in the previous e-mail: there is no
course credit for the homework. One purpose of the homework
is to prepare for the exams. This is surely more effective if you
are free to consult each other about the solutions.
--- ... B..

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From: me
To: Mr. B
Subject: [ I couldn't find the topic in the emails, i must have deleted the original one]

sorry,
thanks for warning
fatih olmez

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From: Mr. B
To: me
Subject: bir $ey degil [ it is in Turkish and means "that's all right" ]

Fatih,
No problem.
My own flaw in e-mail correspondence is that I
tend to be ironic without using the symbol :-)
--- ...