Dear Ms Rossi,
'Gentile' = jen-TI-le; the final 'e' is always pronounced in Italian.
Use as the opening salutation in any formal email to a woman whose name you know. It is the safest and most universally accepted greeting in Italian professional writing.
'Gentile' literally means 'kind' but functions as 'Dear' in correspondence. 'Signora' covers both 'Mrs' and 'Ms', making it ideal when marital status is unknown. For men use 'Signor' (no final 'e'). For doctors or professors, replace with their title: 'Gentile Dottoressa Rossi'.
Egregio Signor Bianchi,
Dear Mr Bianchi,
Even more formal; used for officials or senior executives
Gentili Signori,
Dear Sirs/Dear all,
When writing to a company or a group
Buongiorno,
Good morning,
Neutral opening for semi-formal emails within a company
In Italy, professional titles (Dottore, Ingegnere, Avvocato) are used far more routinely in correspondence than in English-speaking countries. When in doubt, use the title — omitting it can be perceived as disrespectful.