What is the permissible (non-haram) form of addressing a non-Muslim in writing, especially when dealing with foreign companies where phrases like "Dear Sir" or "Dear Phil" are used? And what is the form if the addressee's religion is unknown?

It is impermissible to initiate greetings of peace (salam) with Jews, Christians, and other non-Muslims, nor to address them with terms of glorification such as "Sayyid" (Master) or "Dear Sir," due to the clear prohibitions established in the Sunnah regarding this. However, they can be addressed by their professional titles, such as "Engineer" or "Accountant." If the religion of the addressed person is unknown, they should be treated according to the predominant assumption: if it is highly probable that they are Muslim, one may initiate the greeting of peace; but if it is highly probable that they are not Muslim, one should not initiate the greeting of peace or use terms that glorify them.