Question: Is it disbelief to say sir to a non-Muslim?
It is disbelief to insult what we have been ordered to show reverence to or to show reverence to what we have been ordered to despise. (The Annotation to Birgivi Vasiyyetnamesi)
It is stated in a hadith-i-sharif: Do not say sir as you talk with a hypocrite [and all kinds of disbelievers]. Showing respect to a disbeliever (kafir) or an oppressor or greeting him respectfully or saying phrases like My master to him causes disbelief. (Bariqa)
It is disbelief to say sir to him out of respect, but it is not disbelief to address him with customary titles. It is disbelief, too, to give salam (Muslim greeting) to a non-Muslim because salam is a dua (supplication, prayer), and one is not allowed to make a dua for a disbeliever. However, one can greet a disbeliever customarily when one needs his help. There is nothing wrong with saying such expressions as bonjour, good morning, or guten Morgen.
So is the case with titles like Mister, Mistress, Madam, and Mademoiselle . Expressions like Sincerely or Dear
are used at the end or beginning of e-mails and letters. They are used as they are customary. Besides, it is permissible to use them to avoid saddening a non-Muslim.