Omitting subject in formal business letters: "We thank you for.." vs. "Thank you for.."

Question

Which is better in a formal letter?

Dear Sirs,
Thank you for the information contained in your letter dated 18 September of this year, and also for the catalogue enclosed with your letter.

or

Dear Sirs,
We thank you for the information contained in your letter dated 18 September of this year, and also for the catalogue enclosed with your letter.

or

Dear Sirs,
Our company thanks you for the information contained in your letter dated 18 September of this year, and also for the catalogue enclosed with your letter.

I was told by a (Russian-native) English business writing teacher that we should not drop subjects in formal business letters. Is this true?

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Regarding the comment:

I would get a better definition of "formal" from your teacher.

I believe that by 'formal' she meant 'any kind of letter between two business entities'. Any letter on business matters.

Answer 1

In the Cambridge Encyclopedia of the English Language (2003), thanks is considered a minor sentence, or more specifically, a type of minor sentence that is a "formula for stereotyped social situations."

In chapter 8 of "An Advanced English Grammar with Exercises" (1913):

Good usage does not demand that all sentences shall be absolutely complete. It often allows (and sometimes requires) the omission of words that, though necessary to the construction, are so easily supplied by the mind that it would be mere waste of time to utter them.

The first example of such an elliptical sentence is:

[I] thank you.

 

"Thank you" without an explicit subject has been used since at least the 15th century, according to an article in the ICAME Journal (vol. 26:63–80):

Interestingly, the shift from a clearly performative speech act with a subject, e.g. I thank you or I give thanks to you to the shortened forms thank you and thanks, had not gone far in Early Modern English. The shortened forms appeared in the 15th century and there are only five examples in my material [Corpus of English Dialogues].

 

So, although it is generally regarded as grammatically correct, it might sound too informal for business communications. That is entirely subjective, though. Of the alternatives you provided, I think the one beginning with "we" sounds best (see edit below):

We thank you for the information...

Alternatives to thank you can also give the letter a more professional tone:

We appreciate the information...

We are [very] appreciative of the information...

Our company is [very] grateful for the information...

 

Edit:
Rather than rely on my own assumptions, I tried to find more references for business writing etiquette with regard to thank-you letters or letters of appreciation. I think now that my opinion was wrong about the implied subject for thank you being too informal for business communications.

I found a reference from the American Management Association, The AMA Handbook of Business Writing (2010), which gives examples of several types of business letters. In it, there is an 8:3 ratio in favor of using the implied subject form, "Thank you," rather than for example, "I want to personally thank you..."

In another book with business letter examples, 300+ Successful Business Letters for All Occasions, the first example of a Thank-you letter (pg 51), begins with "Thank you..." The second example on page 52 also starts with the implied subject form, although the last sentence uses an explicit subject "We thank you..." The examples of other types of letters often use of the implied subject form, too.

Answer 2

I think you should go with

Dear Sirs,

We thank you for the information contained in your letter dated 18 September of this year, and also for the catalogue enclosed with your letter.

The We used as subject in the second case gives a sense of belonging to people, so it could be more effective. In the other two cases where the company and no subject is used, this factor is missing and could be less effective.

Secondly, It won't be a big difference but If you don't have other things to thank for you can make it slightly different like

Dear Sirs,

We thank you for the information contained in your letter dated 18 September of this year. We are also grateful for the catalogue enclosed with your letter.

So using grateful as another word of appreciation besides thankful will make it richer and it will also sound pleasant. And now the sentence is even more courteous. But, again, that depends on your letter.

Answer 3

I also believe in formal business it's better not to drop the subject. But your first sentence is accepted in formal situations as well -

Dear Sirs,
Thank you for the information contained in your letter dated 18 September of this year, and also for the catalogue enclosed with your letter.

Why? Because that thank in Thank you as appears in that quoted sentence is not a verb. Thank you there is an exclamation. Or as Macmillan Dictionary says it's an interjection -

On the other hand in the following quotations thank in thank you is a verb, and you is it's object.

Dear Sirs,
We thank you for the information contained in your letter dated 18 September of this year, and also for the catalogue enclosed with your letter.

Or

Dear Sirs,
Our company thanks you for the information contained in your letter dated 18 September of this year, and also for the catalogue enclosed with your letter.

Hence I think this one is not accepted in formal situations -

Dear Sir
Thanks you for ....

Here we need a subject. Thank you is an interjection, thanks you is not.

Answer 4

The "subject" line in a business letter was intended for secretaries or mail-room personnel who would read it and either file it or direct the letter to the proper department. That way they did not need read the entire letter or even know about what the letter discussed.

Hardly anyone has secretaries these days, so while a subject is still important, you can be less formal and assume that the letter will be read directly by the recipient unless you are writing to someone very high up who does have an admin-assistant.
This leads us to the only conclusion - be direct and simple. This leaves me with only one choice, your first entry.

Dear Sirs, Thank you for the information contained in your letter dated 18 September of this year, and also for the catalogue enclosed with your letter.

If this is an assignment and your teacher has a prescribed format, then everything I said goes out the window. Follow his/her format. Hope this helps.