According to the email, the iPhone 6s is expected to be released come 25th of September in the UK. Pre-orders are also expected to go live a week before on the 18th of September. Now given that the US and UK are part of the same group of markets that are usually among the first to get the device, it is safe to say that the 25th of September release date can be applied to the US market as well.
That being said there was no mention of the larger iPhone 6s Plus which may or may not be released on the same date, but here’s hoping that it will. Now it is possible that the employee was just speculating the dates since we are several months away from a confirmation. However if you recall last year May Deutsche Telekom told its customers to expect the iPhone 6 on the 19th of September, and it turned out the date was true.
Perhaps Apple has started to notify its carrier partners in advance to allow them to better prepare for the launch, which is why this rumor has once again surfaced around the same time this year. Then again it could simply be a coincidence, but either way take it with a grain of salt for now.