Please have another look on my previous email.

To me, all of them can be used.

Hi, I have a question related to the original question.
Which one of the mentioned sentences is the most appropriate in the following situations:
1) Formal business letter (to a co-worker)
2) When talking to an old friend (bestfriend for example)

Thanks

2 is formal. As for 1 and 3 I can't state ant difference between them but I don't want to say that they are informal.

Hi guys,

You don't look on an email. You look at an email.

Best wishes, Clive

It is really a shame that we missed that point. Thanks for clearing it out Clive.

I really don't think anyone would ever actually use 'relook' as a noun.

CliveHi guys,

You don't look on an email. You look at an email.

Best wishes, Clive

Hi Clive,

May I know the reason for that?

>I really don't think anyone would ever actually use 'relook' as a noun.

Agree.

Hi,

You don't look on an email. You look at an email.

May I know the reason for that?

'At' is idiomatic. We like to say that in such a context..

Here are a couple of further comments. I'm not sure if they will help you or just confuse you.

CliveYou might say 'Look at the second line on this email, meaning "look at this particular place on the email".

Hi Clive,

How about 'in'? Would 'Look at the second line in this email' be okay?

Thanks,
Hoa Thai