Should I use both surnames when addressing a Spanish professor as 'Dr' in an email? I'm writing my professor. She has two surnames. So far, I've been using only her first surname; however, I'm curious whether that's, generally, how I should address a professor with two surnames. I don't know whether the addressee's culture affects the answer, but the professor is from Spanish South-America. (I've noticed that some Spanish people use only the first of both last names, but most English people who take two names ask others to use both names.) Also, I don't whether familiarity matters but we communicate frequently enough that I begin my emails with 'Hi Dr...' instead of 'Dear Dr...'. 

Thank you
Mexican here!!

We also have two surnames, and to be perfectly honest we do not mind if you use both or only one of our last names. 

People call me Dr Palafox and my former advisor (Who is the head of the CS department) Dr Benitez. We are talking people from all the spectrum of workers, from administrative and cleaning staff to general directors. So is perfectly fine to use only one last name. 

Actually is going to sound weird if you end up using both names, because we rarely do. Even the President is rarely called by his 2 last names, unless is a very specific situation.