The opening sentence of a cover letter comes after the greeting and acts like a title or subject line. It should sum up why you are contacting the recipient, and begin to introduce what the letter contains.
You can reference previous contact you have had with the recipient in this section. For example, you can mention the connection if you were introduced or referred by a mutual colleague, or previously met the recipient.
Here is an example of an opening sentence:
"Per our previous discussion, enclosed is our proposal to provide business consulting services for your business."
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