With an example:
I want to write the mail an important person, with this form
I want to say to the formal salute, or if I use the Hi/Hello don't is an error?
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Salutations in English are formulaic. That is, you cannot change your actual greeting very much from the greeting expected by the reader.
This means you will use the following very often: Dear honorific surname, though you may use Dear given name surname, if it is necessary to avoid confusion or it is desirable to avoid using the honorific. A title may also be used in place of the honorific.
Dear Dr Williams,
Dear Elizabeth Johnson,
Dear Ms Watson,
Dear Mr Smith,
Dear Professor Black,
And so on. Note the lack of punctuation at the end of abbreviated honorifics. This is current British usage.