I have been sending emails (in English) to the "visitor's program" of an institute in Germany. Initially, only one person used to reply to these emails, and I addressed this person.
Recently, two more people started replying from the same account (and they seem to reply in some random order). I started addressing the person who last replied, but since then, it has happened twice that I addressed an email to "Mr. A", but "Ms. B" or "Mr. C" replied to that email.
Also, some of them write
Dr. in their signature, while some do not write. And they are not professors (So Dear Prof. A, Prof. B, Prof. C won't work). Since some of them don't write Dr., I feel it may not be a good idea to write "Dear Mr. A, Ms. B, and Dr. C".
Can I write "To whom it may concern"? Is that somehow considered rude?
Although this institute is in Germany, feel free to add information about other countries.
I don't think this is a duplicate of this because I am concerned about addressing more than one people simultaneously, while that question seems to be more concerned about gender of the recipient.