Capital letters are very important but nowadays they are being left out more and more frequently. One reason is that because people do not use capital letters in their email address, they sometimes wrongly believe that it is acceptable to leave them out of a postal address. It is not unusual for employers to receive application letters with no capital letters in the address. This is a significant mistake and would not encourage any employer to offer such people a job.
Capitals must always be used at the start of a sentence and the first word of a letter (Dear John. When I saw your letter …). They must also be used for the names of people (Jacob Morley) as well as their title (Mrs Ms Mr), mountains (Mount Kilimanjaro), countries (Sri Lanka), capital cities (London), the first word of book titles at the very least (In Search of Fatima) as well as important buildings such as the House of Commons or Westminster Abbey. We must also use them when we speak about populations (the Chinese, the English) and religions (Christianity, Islam, Sikhism, the Quakers).
We use them when we refer to regions of the world (the East) or to areas (West Lothian; West Midlands) but not when we are giving directions (Turn east after the river…). We use them in titles (Dr Liu; Professor Kanji) but not when we are speaking generally (It’s clear that doctors follow a rigorous training course). We use capitals to refer to dates (B.C or A.D) but not to time (a.m. and p.m.). We use capitals for months (August) and days (Tuesday) but not to illnesses (influenza. chickenpox).
We use capitals for some senior titles and positions (Lord Giddens, The Speaker of the House of Commons, the Chief Whip, the Head, the Principal) but not in general (I think that teachers work very hard. She thinks that fire-fighters are brave.).
People sometimes use capitals in emails to highlight something. However, when it is used throughout an email it is seen as impolite and ‘shouting’ and so many people will avoid it.