Don't waste people's time It is polite to reply to e-mails promptly. A few words suffice as a holding reply (and reassure the sender the e-mail has been received), until a longer reply can be composed. Use Proper Punctuation

Ensure that correct punctuation is used, and do not succumb to the habit of using lowercase letters throughout.

Avoid writing a pointless subject field

The subject line is a summary of the content of the e-mail, and should alert the recipient. A well-written subject line will ensure that the e-mail gets the appropriate attention.

Should you send a letter instead?

Written correspondence must never be replied to solely by e-mail. If an urgent response is necessary, then a telephone call or an e-mail is acceptable, but only provided it is followed up with the appropriate written correspondence.

Include your contact details

Business e-mail signatures should supply relevant information, such as job title, company website address, telephone and e-mail address.

Salutation and sign-off

Suggested informal salutation: name only (ie, "John"). Informal sign-off: "See you soon." Formal salutation: "Dear Mr Davenport." Informal sign-off: "Kind regards." Err towards the formal.

Source: Debrett's, Correct Form