Il ne faut pas confondre for et during. For répond à la question how long? (combien de temps ? ; pour combien de temps ?) (I went to Boston for three weeks), alors que during répond à la question when? (quand ?) (I went to Boston during the holidays).
In e-mails you can use any of the standard phrases you use when starting or ending a letter, however it is not necessary to include these. When replying to an e-mail, even in a professional context, it is common to miss out any greeting at the start or end of the message. You can use informal language when e-mailing friends, but in a professional context, or if you are writing to someone you do not know very well or do not know at all, it is better to be more formal.