In writing to academics and scientists, perhaps the solution is simple enough: use Dear Dr Tanaka or Dr Smith or whatever surname or family name your correspondent has—if he or she has a doctorate degree, you have used the correct form of address; if not, the person is unlikely to mind.
However, a common convention in addressing individuals when you do not know whether you are writing to a woman or a man is to use both the first name and the last name or the surname, without using a title , as in Dear Pat McNees or Hello Lesley Smith . However, this form might not be acceptable with initials instead of the first name; Dear A J Cronin , for example, might sound odd.
If you are writing to people when you do not even know their names, let alone their gender, it is acceptable to address the letter to a job title or designation, as in Dear Manager or Dear Editor or Dear President as the case may be, so long as you know the correct designation. Incidentally, it is customary in the US to end the salutation with a colon in formal correspondence (as in Dear Dr Smith: ), whereas in the UK, a comma is more common (as in Dear Dr Smith, ).