We have a minefield here.
In British and American usage it is safest, in the first email, to address people you have not met by their title and surname/last name/family name, especially if they are in a higher position than you are -
Dear Mr/Mrs/Ms/Miss/Dr/Professor(One f, two esses) /Senator/General Tameme.
If you have met them, then address them in the way in which you have spoken to them.
If you are writing to a student/employee/ someone who is in a lower position than you, it is normally acceptable to use the first name - Dear Ihab.
If you are employed, find out what company/university/institution policy is.
If in doubt, go for title + last name. You may be considered a little old-fashioned, but at least you won't be considered rude.
- For Teachers