Question: Is it disbelief to say “sir” to a non-Muslim?
ANSWER
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 du’a (supplication, prayer), and one is not allowed to make a du’a 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.