You successfully passed a job interview. You are expected to start on November 15, but you

You successfully passed a job interview. You are expected to start on November 15, but you will not be available on that date.
Task:
Write a letter to your new boss,
explaining your situation,
expressing your concern and suggesting a solution.

Dear Sir or Madam

I am Mahdi Hajirajabi ,the network engineer who passed a job interview in your company for working at penetration test department on 23 October.I am writing this letter to inform you that I will not be able to start my new job as a network security expert this week.

As I agreed in my employment contract, I should be at work 2 weeks after my job interview, in this period I could complete my activities at the current job and move to your company .Unfortunately I caught a cold ,then I had to stay at home under medical cares , so going to the office was impractical for me.

Although it was an unavoidable issue, I sincerely apologize for all the possible inconveniences that may have been occurred because of my absence. However, I attend in your office to initiate my new career in the beginning of the next week .In addition , in this period I can accomplish my tasks at home through Internet.

Please let me know if you need any proof of my disease or I should fill some forms in this regard .It would be my honor and a rewarding experience to be a part of your team.

Yours faithfully
Mahdi Hajirajabi

Dear Sir/ Madam


.... Generally, you receive a letter from the employer informing that you've been selected and to report to work on a particular day. So, you need to write back to this person about your inability to make it on that day. So, you need not to mention about the tests because he or she already knows.

First, thank you very much for informing me that I have been selected to work as a Network Engineer at your prestige institution. I am delighted with this job offer and would really looking forward to making my fullest contribution to your organization. However, due to some unavoidable commitment, I have an issue with the date, 23rd October, that you requested me to report to work.

I'm really thankful for the corrections you made, as I understood you mean nothing should be written to help my boss to remember me?

Well.... I am too with Pahan because this is a job contract and you have to assume that you received a letter from them requesting you to report on that day. So in this case you have to refer to the letter first. Anyway you will be signing off at the end of the letter with your name and that will help them to identify you. Also, since this is an official letter, you can have a heading like;

Position of Network Engineer ... that may help them connect the reason why you are writing this letter.