I disagree @Vriddle. If you don't know a person's title, the first contact is Dear Jane Smith, signed off as John Brown. If a title is offered, you then use it.
If no title were offered, I'd write the next email as "Dear Jane (if I may).
I work in a sector riven by titles and everyone is first name with the title at the footer only. My footer says Mrs. and gives my job title.
Unknown
Jane Smith
Head of Gymnastics
Dear Jane Smith
Yours sincerely
Julie Brown
Known and formal
Mrs Jane Smith
Head of Gymnastics
Dear Mrs Smith
Yours sincerely
Julie Brown (Ms)
Known and informal
Mrs Jane Smith
Dear Jane
Yours sincerely
Julie
Rude and inappropriate
Mary White
Dear Mary or Mary White
Yours sincerely
Dr James Price
Nobody should fail to afford another the same level of courtesy they expect to receive.
A High Court Judge would refer to the defendant in the Dock as Miss, Ms, Mrs, Mr Jones. Not as first name, surname.