这个范围有点大,给你举个例子,比如“请尽快回信”这句话:
Please reply as soon as possible.
Please reply at your earliest convenience.
Your early reply will be highly appreciated.
明显第三种表达更让人舒服。简单两点:少用、不用祈使句;给邮件对象更多的选择余地。
建议你去买/借一本商务英文写作的书,里面会有详细的指导。