Dear [STRIKE]Mr Manager[/STRIKE] Sir (or his actual name if you know it),
[STRIKE]We just wanted to let you know[/STRIKE] Please note that we will be performing maintenance on the network infrastructure this coming Friday, which will require us to take down the form processing Switches & PBX. Therefore kindly arrange for Mr John (is John his first name or his surname? If it's his first name, then don't use "Mr". If it's his surname, then it's fine) to attend (Our Office Name) on [STRIKE]below mentioned date [/STRIKE] the date and time shown below. The scheduled outage is to happen on :
Friday, August 13th, 2010 8.00 AM / 10.00 AM (do you mean "from 8am until 10am" or "at either 8am or 10am"?)
Regards