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This sentence WILL immediately imply that its not from Microsoft at all because you will have to reply if you have questions and they Micosoft WILL have to answer them. Its impossible that Micorsoft will ever say this. The sentence structure is also written in a more informal and causal form. Micosoft Inc. as a whole will never write informal sentences. Even if its Bill Gates writting to you alone unless you are his very good friend.

this is the latest version of security update, the "September 2003, Cumulative Patch" update which eliminates all known security vulnerabilities affecting MS Internet Explorer, MS Outlook and MS Outlook Express as well as three newly discovered vulnerabilities. Install now to help protect your computer from these vulnerabilities, the most serious of which could allow an malicious user to run executable on your computer. This update includes the functionality of all previously released patches.
This is the main giveaway, never in my life have I ever seen such things claiming that it will patch all vunerabilities because firstly, there is always undiscovered loopholes and secondly, Micosoft's software, especially Windows IS always full of loopholes. Maybe cover all previously found loopholes but never ALL so much so that it makes the program perfect.

Also, the information given is very vague. Microsoft will never just say its just a September cumulative patch. They will always give the serial number of the patch like Q327979.

Thank God that I do pay attention during English Lessons. Its become a force of habit to read all my email this way. I am always very cautious of viruses. Good thing you consulted the forums first.