What is the Aadaab relating to thanking someone (Muslim or Kuffar)? Can we say: "thank you" even though all praise is for Allah (SWT)? Also, how should we address the non-Muslims (men and women) in letters and emails, e.g. when applying for employment? For instance, can we address them with the word 'Dear' etc. even though they are not really dear to us?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

The Prophet (Sallallahu Alaihi wa Sallam) forbade us from glorifying the Kuffar by any kind of exaltation, for instance to reserve the front seats for them and let them sit at the front of any meetings.  We should not greet them first.

Abu Hurairah narrated that the Prophet (Sallallahu Alaihi wa Sallam) said: “Do not start greeting (saying Salaam to) the Jews and Christians.  If you meet one of them in the street, force him into the narrow side”.

If someone writes a letter to a Kafir, he has to start with a general meaning.

The author of Kashaf al-Qina’ stated: ‘If a Muslim writes a letter to a Kafir and writes ‘Salaam’ on it, he should write ‘Salaam to those who follow the guidance’, because this is a general meaning’.

It is not permissible to start the letter by writing anything that could mean the glorification of a Kafir by the Muslim, like the title ‘Sir’ or ‘Dear’ and the like.

He is not your ‘master’ or ‘dear’ while he is declaring Allah to be his enemy.

You can mention instead some academic or work titles that are commonly used when addressing someone, like ‘Doctor’ or ‘Director/Manager’; there is no harm in this, Insha Allah.

It is reported in an authentic Hadith that the Prophet (Sallallahu Alaihi wa Sallam) wrote to Heraclius and stated in his letter ‘To Heraclius, the greatest of Rome’.

If a Kafir does something good to a Muslim, then there is no harm in rewarding him for his good deed, but the Muslim should not reward him with something that could imply glorification.

The Muslim could offer him something, and it is permissible for him to make Du’a to Allah to guide him and honour him.

The author of Matalib Ulli al-Nuha, said: ‘It is permissible to say ‘May Allah make you more honourable and guide you, meaning guide you to Islam’.

Ibrahim Al Harbi said to Ahmad (may Allah have mercy on both of them):  ‘The Muslim can say to a non-Muslim who did him something good: “May Allah make you more honourable”'.  Ahmed replied, “Yes, meaning by making Muslim”.

What the questioner mentioned that “All the praises and thanks are due to Allah alone” is correct. But it is not Allah only who should be thanked or praised, but we have to thank anyone who does good to us, whoever he might be.

Allah, the Exalted has ordered us to be thankful to our parents after being thankful to Him.  Allah Says (interpretation of meaning): {….give thanks to Me and to your parents, unto Me is the final destination.}[31:14].

The Prophet (Sallallahu Alaihi wa Sallam) dispraised a woman who was not thankful to her husband.  We must thank and compensate whoever does good to us.  The texts about rewarding a person who does good deed to someone are general and not specific.

The Prophet (Sallallahu Alaihi wa Sallam) said: “You have to reward anyone who does good to you, if you do not find anything to reward him with, then make Du’a for him until you realize that you have rewarded him”. [Reported by Abu Dawud and others]

Allah knows best.