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Adding a few aspects to the very good responses above:

I'm a German professor in CS and I prefer "Herr xxx" as well. In fact I'm becoming suspicious if a student adresses me with "Professor xxx" in spoken conversation ;-). For written conversation of unknown students, it would be ok. It doesn't matter to me if it's "Sehr geehrter Herr Prof. xxx" or "Sehr geehrter Prof xxx".

But this is me and it is CS! I assume, that professors who are active in SE are more open-minded then others, so answers here might be biased.

The answer is completely different, when you are talking about law, economics or medicine. Here it is much more likely to find professors who are really obsessed about titles and it would be a huge no-go not to use all available titles in written conversation. Better check their website in advance. Really.

In the medical field, "Herr Professor" without last name is quite common, even in spoken conversation.

Fun fact: You can have your "Dr." title in your passport, but not the "Prof" title, so my passport says "Dr. XXX".

Adding a few aspects to the very good responses above:

I'm a German professor in CS and I prefer "Herr xxx" as well. In fact I'm becoming suspicious if a student adresses me with "Professor xxx" in spoken conversation ;-). For written conversation of unknown students, it would be ok. It doesn't matter to me if it's "Sehr geehrter Herr Prof. xxx" or "Sehr geehrter Prof xxx".

But this is me and it is CS! I assume, that professors who are active in SE are more open-minded then others, so answers here might be biased.

The answer is completely different, when you are talking about law, economics or medicine. Here it is much more likely to find professors who are really obsessed about titles and it would be a huge no-go not to use all available titles in written conversation. Better check their website in advance. Really.

Fun fact: You can have your "Dr." title in your passport, but not the "Prof" title, so my passport says "Dr. XXX".

Adding a few aspects to the very good responses above:

I'm a German professor in CS and I prefer "Herr xxx" as well. In fact I'm becoming suspicious if a student adresses me with "Professor xxx" in spoken conversation ;-). For written conversation of unknown students, it would be ok. It doesn't matter to me if it's "Sehr geehrter Herr Prof. xxx" or "Sehr geehrter Prof xxx".

But this is me and it is CS! I assume, that professors who are active in SE are more open-minded then others, so answers here might be biased.

The answer is completely different, when you are talking about law, economics or medicine. Here it is much more likely to find professors who are really obsessed about titles and it would be a huge no-go not to use all available titles in written conversation. Better check their website in advance. Really.

In the medical field, "Herr Professor" without last name is quite common, even in spoken conversation.

Fun fact: You can have your "Dr." title in your passport, but not the "Prof" title, so my passport says "Dr. XXX".

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source | link

Adding a few aspects to the very good responses above:

I'm a German professor in CS and I prefer "Herr xxx" as well. In fact I'm becoming suspicious if a student adresses me with "Professor xxx" in spoken conversation ;-). For written conversation of unknown students, it would be ok. It doesn't matter to me if it's "Sehr geehrter Herr Prof. xxx" or "Sehr geehrter Prof xxx".

But this is me and it is CS! I assume, that professors who are active in SE are more open-minded then others, so answers here might be biased.

The answer is completely different, when you are talking about law, economics or medicine. Here it is much more likely to find professors who are really obsessed about titles and it would be a huge no-go not to use all available titles in written conversation. Better check their website in advance. Really.

Fun fact: You can have your "Dr." title in your passport, but not the "Prof" title, so my passport says "Dr. XXX".