Clarify that this email is a reply to a business proposal on the headline. It is not mandatory to mention your decision too, but it is helpful to avoid misunderstandings.
Mention your company's details at the top of the email to make it clear to the recipient that this is an official reply. If applicable, include a company logo.
State the recipient's name, designation and his or her company name. It is acceptable to begin the main part of the letter with "Dear" after you have mentioned these details.
Clarify your intentions to support the proposal in the first paragraph. The reader must be absolutely sure whether you have accepted the offer or not by the time he or she moves to the next paragraph.
Ask for clarification on specific issues of the proposal. Mention their position in the original offer or attach pictures of the offer with the ambiguous spots circled.
Inform the recipient of what's happening next. For example, ask him or her to contact you or prompt the recipient to move on with any preparations.
Conclude the main part of the email by mentioning your name, designation, company name and contact details, including phone and fax number.
Scan any attachments for viruses, and make sure they are in a commonly used format (.jpeg for images, .docx for documents, etc.). Compress files that are too large to include using programs such as WinRAR.