How to address people in emails/letters

Hallo,
I always wonder what the appropriate way of addressing people in emails is. Being German, I'm not feeling very comfortable in addressing other people by their first name if I don't know them personally. Especially with persons more senior than myself I prefer to err on the wrong side and be rather more formal (and write 'Dear $title $lastname' and end with 'Yours sincerely' or something similar), even if they address me with my first name and sign with their first name.

Now I've been told that some people may consider it rude if this isn't reciprocated. Is there any general rule which applies on this issue? Is it safe to remain on the formal side? Or is it better to be less formal as soon as the 'counterpart' starts to be less formal?

Thank you for your help,
Gunter