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Always use the date your are writing the letter as the date of the letter. UNLESS, you are writing the letter on a date before when you will send the letter. In this case, use the date that the letter will actually be sent.
When addressing multiple couples in a formal letter, you can use "Dear Mr. and Mrs. Smith, Mr. and Mrs. Johnson," or simply "Dear Mr. and Mrs. Smith, and Mr. and Mrs. Johnson." If the couples have different last names, ensure you list each couple appropriately. For a more inclusive option, you can also use "Dear All" if the context allows for a more casual tone.
The proper way to address a female boss in writing is to use Dear Miss and then the name. If she is married then use Ms. or Mrs. instead.
Yes
Most people Understand
Use the prefix of Mrs. if it is a fomal letter. If not, and you are familiar with them, use their first name.
When writing a recommendation letter, you should use the salutation "Dear Recipient's Name" to address the person you are recommending.
What ever you want to use
When writing a letter, it is appropriate to use Mr.
Since Women's Liberation came into play during the 1960s some women do not want to be titled as Mrs. or Miss so they have shortened it for both married to single women to Ms. It is not that common now to use 'Ms' but a good idea to use it in a letter if one does not know if the person they are writing to is a Mrs. or a Miss.
Calligraphy is used for fancy writing for like a love letter or just a letter
You are writing a business letter, you should be more formal.